Hi Fabian,
Here you are... pbs server is a dedicated machine with 16G ram (i7) and multi 10G interfaces...
INFO: starting new backup job: vzdump --mailnotification always --storage zz_PBS01 --node burfell --all 1 --quiet 1 --mode snapshot
INFO: Starting Backup of VM 103 (qemu)
INFO: Backup started at 2021-03-28 06:00:02
INFO: status = stopped
INFO: backup mode: stop
INFO: ionice priority: 7
INFO: VM Name: ***
INFO: include disk 'virtio0' 'esnx-a-core:103/vm-103-disk-0.raw' 16G
INFO: creating Proxmox Backup Server archive 'vm/103/2021-03-28T04:00:02Z'
INFO: starting kvm to execute backup task
INFO: started backup task '1a68d8b1-3553-441f-921b-9a71465b7515'
INFO: virtio0: dirty-bitmap status: created new
INFO: 3% (644.0 MiB of 16.0 GiB) in 3s, read: 214.7 MiB/s, write: 0 B/s
INFO: 98% (15.7 GiB of 16.0 GiB) in 2m 4s, read: 337.3 MiB/s, write: 0 B/s
INFO: 100% (16.0 GiB of 16.0 GiB) in 2m 6s, read: 142.0 MiB/s, write: 0 B/s
INFO: backup is sparse: 13.99 GiB (87%) total zero data
INFO: backup was done incrementally, reused 16.00 GiB (100%)
INFO: transferred 16.00 GiB in 126 seconds (130.0 MiB/s)
INFO: stopping kvm after backup task
INFO: running 'proxmox-backup-client prune' for 'vm/103'
INFO: Finished Backup of VM 103 (00:02:11)
INFO: Backup finished at 2021-03-28 06:02:13
INFO: Starting Backup of VM 119 (qemu)
INFO: Backup started at 2021-03-28 06:02:13
INFO: status = stopped
INFO: backup mode: stop
INFO: ionice priority: 7
INFO: VM Name: ***
[....]
INFO: Starting backup: ct/9031/2021-03-28T07:19:03Z
INFO: Client name: burfell
INFO: Starting backup protocol: Sun Mar 28 09:19:59 2021
INFO: Downloading previous manifest (Sun Mar 21 08:17:06 2021)
INFO: Upload config file '/var/tmp/vzdumptmp5828_9031/etc/vzdump/pct.conf' to 'pbscnr@pbs@172.29.233.254:8007
bs01' as pct.conf.blob
INFO: Upload directory '/var/tmp/vzdumptmp5828_9031' to 'pbscnr@pbs@172.29.233.254:8007
bs01' as root.pxar.didx
INFO: root.pxar: had to upload 2.00 GiB of 8.61 GiB in 465.92s, average speed 4.39 MiB/s).
INFO: root.pxar: backup was done incrementally, reused 6.61 GiB (76.8%)
INFO: Uploaded backup catalog (845.53 KiB)
INFO: Duration: 469.04s
INFO: End Time: Sun Mar 28 09:27:48 2021
ERROR: Backup of VM 9031 failed - proxmox-backup-client failed: Error: http request timed out
INFO: Failed at 2021-03-28 09:28:11
INFO: Starting Backup of VM 9115 (qemu)
INFO: Backup started at 2021-03-28 09:28:11
INFO: status = running
INFO: VM Name: ***
INFO: include disk 'virtio0' 'esns-b:9115/vm-9115-disk-1.qcow2' 100G
INFO: include disk 'virtio1' 'esnx-a:9115/vm-9115-disk-1.qcow2' 32G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/9115/2021-03-28T07:28:11Z'
INFO: skipping guest-agent 'fs-freeze', agent configured but not running?
INFO: started backup task '6cbe2d3c-8801-4672-bb74-784617743cee'
INFO: resuming VM again
INFO: virtio0: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: virtio1: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: 0% (196.0 MiB of 132.0 GiB) in 3s, read: 65.3 MiB/s, write: 1.3 MiB/s
INFO: 1% (1.4 GiB of 132.0 GiB) in 20s, read: 71.5 MiB/s, write: 0 B/s
INFO: 99% (131.5 GiB of 132.0 GiB) in 1h 11m 51s, read: 1.4 GiB/s, write: 10.7 MiB/s
INFO: 100% (132.0 GiB of 132.0 GiB) in 1h 12m 22s, read: 17.0 MiB/s, write: 4.4 MiB/s
INFO: backup is sparse: 61.48 GiB (46%) total zero data
INFO: backup was done incrementally, reused 116.83 GiB (88%)
INFO: transferred 132.00 GiB in 4347 seconds (31.1 MiB/s)
ERROR: Backup of VM 9115 failed - proxmox-backup-client failed: Error: http request timed out
INFO: Failed at 2021-03-28 10:41:08
INFO: Starting Backup of VM 9117 (qemu)
INFO: Backup started at 2021-03-28 10:41:08
INFO: status = running
INFO: VM Name: ***
INFO: include disk 'virtio0' 'esnx-a:9117/vm-9117-disk-0.raw' 12G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/9117/2021-03-28T08:41:08Z'
INFO: started backup task 'dfde0caa-14da-4479-8751-d4bcccb0ada1'
INFO: resuming VM again
INFO: virtio0: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: 1% (184.0 MiB of 12.0 GiB) in 3s, read: 61.3 MiB/s, write: 5.3 MiB/s
INFO: 2% (264.0 MiB of 12.0 GiB) in 6s, read: 26.7 MiB/s, write: 0 B/s
INFO: 99% (11.9 GiB of 12.0 GiB) in 7m 20s, read: 32.0 MiB/s, write: 0 B/s
INFO: 100% (12.0 GiB of 12.0 GiB) in 7m 23s, read: 36.0 MiB/s, write: 0 B/s
INFO: backup is sparse: 2.40 GiB (20%) total zero data
INFO: backup was done incrementally, reused 11.59 GiB (96%)
INFO: transferred 12.00 GiB in 443 seconds (27.7 MiB/s)
ERROR: Backup of VM 9117 failed - proxmox-backup-client failed: Error: http request timed out
INFO: Failed at 2021-03-28 10:48:58
INFO: Starting Backup of VM 9119 (qemu)
INFO: Backup started at 2021-03-28 10:48:58
INFO: status = running
INFO: VM Name: ***
INFO: include disk 'virtio0' 'esns-b:9119/vm-9119-disk-0.raw' 32G
INFO: include disk 'virtio1' 'esnx-a:9119/vm-9119-disk-0.raw' 32G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/9119/2021-03-28T08:48:58Z'
INFO: issuing guest-agent 'fs-freeze' command
INFO: issuing guest-agent 'fs-thaw' command
INFO: started backup task '973688b6-195d-432d-8455-cba3cc350cb5'
INFO: resuming VM again
INFO: virtio0: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: virtio1: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: 0% (192.0 MiB of 64.0 GiB) in 3s, read: 64.0 MiB/s, write: 18.7 MiB/s
INFO: 1% (656.0 MiB of 64.0 GiB) in 42s, read: 11.9 MiB/s, write: 2.3 MiB/s
INFO: 99% (63.9 GiB of 64.0 GiB) in 11m 1s, read: 394.7 MiB/s, write: 0 B/s
INFO: 100% (64.0 GiB of 64.0 GiB) in 11m 2s, read: 144.0 MiB/s, write: 0 B/s
INFO: backup is sparse: 47.57 GiB (74%) total zero data
INFO: backup was done incrementally, reused 61.96 GiB (96%)
INFO: transferred 64.00 GiB in 662 seconds (99.0 MiB/s)
ERROR: Backup of VM 9119 failed - proxmox-backup-client failed: Error: http request timed out
INFO: Failed at 2021-03-28 11:00:21
INFO: Starting Backup of VM 9200 (qemu)
INFO: Backup started at 2021-03-28 11:00:21
INFO: status = running
INFO: VM Name: ***
INFO: include disk 'virtio0' 'esns-b:9200/vm-9200-disk-0.raw' 32G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/9200/2021-03-28T09:00:21Z'
INFO: started backup task '71e6afce-d5f4-4697-b69e-51187c630054'
INFO: resuming VM again
INFO: virtio0: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: 2% (672.0 MiB of 32.0 GiB) in 3s, read: 224.0 MiB/s, write: 0 B/s
INFO: 3% (1.2 GiB of 32.0 GiB) in 6s, read: 180.0 MiB/s, write: 2.7 MiB/s
INFO: 98% (31.5 GiB of 32.0 GiB) in 1m 55s, read: 448.0 MiB/s, write: 0 B/s
INFO: 100% (32.0 GiB of 32.0 GiB) in 1m 57s, read: 232.0 MiB/s, write: 0 B/s
INFO: backup is sparse: 29.32 GiB (91%) total zero data
INFO: backup was done incrementally, reused 31.86 GiB (99%)
INFO: transferred 32.00 GiB in 117 seconds (280.1 MiB/s)
INFO: running 'proxmox-backup-client prune' for 'vm/9200'
INFO: Finished Backup of VM 9200 (00:02:02)
INFO: Backup finished at 2021-03-28 11:02:23
INFO: Starting Backup of VM 30000 (qemu)
INFO: Backup started at 2021-03-28 11:02:23
INFO: status = running
INFO: VM Name: ***
INFO: include disk 'virtio0' 'esns-b:30000/vm-30000-disk-1.qcow2' 32G
INFO: include disk 'virtio1' 'esnx-a:30000/vm-30000-disk-0.qcow2' 32G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/30000/2021-03-28T09:02:23Z'
INFO: issuing guest-agent 'fs-freeze' command
INFO: issuing guest-agent 'fs-thaw' command
INFO: started backup task '8c063bc6-114e-45a5-b665-dc9c35d8d354'
INFO: resuming VM again
INFO: virtio0: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: virtio1: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: 0% (96.0 MiB of 64.0 GiB) in 3s, read: 32.0 MiB/s, write: 2.7 MiB/s
INFO: 1% (664.0 MiB of 64.0 GiB) in 25s, read: 25.8 MiB/s, write: 372.4 KiB/s
INFO: 98% (63.0 GiB of 64.0 GiB) in 7m 24s, read: 424.0 MiB/s, write: 0 B/s
INFO: 100% (64.0 GiB of 64.0 GiB) in 7m 27s, read: 344.0 MiB/s, write: 0 B/s
INFO: backup is sparse: 52.23 GiB (81%) total zero data
INFO: backup was done incrementally, reused 62.71 GiB (97%)
INFO: transferred 64.00 GiB in 447 seconds (146.6 MiB/s)
ERROR: Backup of VM 30000 failed - proxmox-backup-client failed: Error: http request timed out
INFO: Failed at 2021-03-28 11:10:11
INFO: Starting Backup of VM 90010 (qemu)
INFO: Backup started at 2021-03-28 11:10:11
INFO: status = running
INFO: VM Name: ***
INFO: include disk 'virtio0' 'esnx-a:90010/vm-90010-disk-0.raw' 64G
INFO: include disk 'virtio1' 'esnx-a:90010/vm-90010-disk-1.raw' 64G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: skip unused drive 'esns-b:90010/vm-90010-disk-0.raw' (not included into backup)
INFO: creating Proxmox Backup Server archive 'vm/90010/2021-03-28T09:10:11Z'
INFO: skipping guest-agent 'fs-freeze', agent configured but not running?
INFO: started backup task '72846bcf-0cd5-4eee-b5ca-b74b31b253e2'
INFO: resuming VM again
INFO: virtio0: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: virtio1: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: 0% (116.0 MiB of 128.0 GiB) in 3s, read: 38.7 MiB/s, write: 8.0 MiB/s
INFO: 1% (1.3 GiB of 128.0 GiB) in 30s, read: 45.0 MiB/s, write: 2.5 MiB/s
INFO: 99% (126.7 GiB of 128.0 GiB) in 54m 30s, read: 406.7 MiB/s, write: 0 B/s
INFO: 100% (128.0 GiB of 128.0 GiB) in 54m 42s, read: 108.7 MiB/s, write: 0 B/s
INFO: backup is sparse: 65.29 GiB (51%) total zero data
INFO: backup was done incrementally, reused 117.14 GiB (91%)
INFO: transferred 128.00 GiB in 3282 seconds (39.9 MiB/s)
ERROR: Backup of VM 90010 failed - proxmox-backup-client failed: Error: http request timed out
INFO: Failed at 2021-03-28 12:05:17
INFO: Starting Backup of VM 90011 (qemu)
INFO: Backup started at 2021-03-28 12:05:17
INFO: status = running
INFO: VM Name: ***
INFO: include disk 'virtio0' 'esnx-a:90011/vm-90011-disk-0.raw' 64G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/90011/2021-03-28T10:05:17Z'
INFO: skipping guest-agent 'fs-freeze', agent configured but not running?
INFO: started backup task 'e3f890b2-8239-4b4f-a4bb-23dbd845ce9c'
INFO: resuming VM again
INFO: virtio0: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: 0% (56.0 MiB of 64.0 GiB) in 3s, read: 18.7 MiB/s, write: 2.7 MiB/s
INFO: 1% (664.0 MiB of 64.0 GiB) in 30s, read: 22.5 MiB/s, write: 6.5 MiB/s
INFO: 99% (63.5 GiB of 64.0 GiB) in 35m 5s, read: 254.7 MiB/s, write: 0 B/s
INFO: 100% (64.0 GiB of 64.0 GiB) in 35m 15s, read: 50.8 MiB/s, write: 0 B/s
INFO: backup is sparse: 18.79 GiB (29%) total zero data
INFO: backup was done incrementally, reused 58.07 GiB (90%)
INFO: transferred 64.00 GiB in 2115 seconds (31.0 MiB/s)
ERROR: Backup of VM 90011 failed - proxmox-backup-client failed: Error: http request timed out
INFO: Failed at 2021-03-28 12:40:56
INFO: Backup job finished with errors
TASK ERROR: job errors
Tobias