I had the same issue with kernel : 4.4.35-1-pve and it was resolved by upgrading to : 4.4.35-2-pve
Tried with a quite large backup afterwards:
INFO: transferred 3543348 MB in 17702 seconds (200 MB/s)
INFO: archive file size: 2836.98GB
INFO: Finished Backup of VM 100 (04:55:06)
INFO: Backup job finished successfully
Problem on our side was with the CEPH nodes. From time to time, OSD daemons where killed and CEPH marked them down. In one morning, half of our OSD was down, cluster was rebuilding, most of the VM partitions where corupted, some of them impossible to recover, data loss, absolutely horror. After the kernel update everything looks ok. Update took place on 3 january, from that time (7 days), a lot of pain... What a mess.
We have the same issue on our server cluster.
Fabian - is this bug fixed in pve-kernel-4.4.35-2-pve that is available using dist-upgrade or it is better to install the version you built?
Thanks in advance!