FYI, this appears to be tracked as bug report #7387 with the same assertion and likely similar backup-path trigger. It may be worth subscribing there for status updates.
https://bugzilla.proxmox.com/show_bug.cgi?id=7387
I found something else in journalctl (on proxmox host) when the VM was stopped:
Apr 11 23:25:40 p** QEMU[4172743]: kvm: ../block/io.c:444: bdrv_drain_assert_idle: Assertion `qatomic_read(&bs->in_flight) == 0' failed.
I may be important, that i...
There is nothing in dmesg VM just suddenly stops without errors inside VM and there is no kernel panic or anything on VNC because VM i stopped not in error state. I have similar VM (the same size, system, configuration...) on PVE8 and they backup...
I have 2 hosts with PVE8 and one with PVE9. Problem is only on PVE9 (9.1.4) and occurs after ~10-12 days from last VM start.
Every day there are 4 backups of VM (schedule: 5,11,17,23:15) which are done from PVE to PBS. Everything works and then...