Error when removing backup for one backup job since a couple of days

[peterge]

...
INFO:  98% (294.1 GiB of 300.0 GiB) in 3h 15m 20s, read: 75.3 MiB/s, write: 67.5 MiB/s
INFO:  99% (297.0 GiB of 300.0 GiB) in 3h 16m 41s, read: 36.3 MiB/s, write: 27.6 MiB/s
INFO: 100% (300.0 GiB of 300.0 GiB) in 3h 17m 45s, read: 47.8 MiB/s, write: 30.8 MiB/s
INFO: backup is sparse: 37.91 GiB (12%) total zero data
INFO: transferred 300.00 GiB in 11865 seconds (25.9 MiB/s)
INFO: archive file size: 207.39GB
INFO: adding notes to backup
INFO: prune older backups with retention: keep-daily=2
INFO: removing backup 'DasiCGN:backup/vzdump-qemu-123-2024_06_28-23_00_02.vma.zst'
ERROR: error when removing backup 'DasiCGN:backup/vzdump-qemu-123-2024_06_28-23_00_02.vma.zst' - removing archive /mnt/pve/DasiCGN/dump/vzdump-qemu-123-2024_06_28-23_00_02.vma.zst failed: Resource temporarily unavailable
ERROR: Backup of VM 123 failed - error pruning backups - check log
INFO: Failed at 2024-07-01 02:22:07
INFO: Backup job finished with errors
TASK ERROR: job errors

Hello, since a couple of days we receive this error in the daily backup job of our VM 123 & 143 (on another proxmox host, so in a different job log). First I checked the other backup jobs and saw that our other backup job starts at 2:00. I changed the other job to start at 3:00, but now this error still shows up.

This is what the Datacenter > Backup page currently looks like:


I do not know why this error keeps showing up, and why it only shows up on two of these VMs & LXCs that get backed up every night.