ERROR: interrupted by signal

Hmm, had a quick glance at the code, but the SIGTERM to the backup job should only be send if a shutdown request is issued to pvescheduler. Not sure why this should be triggered in your case.

I will supplement another observation can somehow help in the investigation of the problem, we also noticed that if you turn off all the tasks in the /etc/pve/jobs.cfg file and start the backup that constantly gets the error error: interrupted by signal - then the backup for this machine is already made without it. Errors, however, if you then turn on all the tasks in this file after that and, for example, in a day, start the backup again, then the backup begins to get an error again, at this moment, having turned off the tasks again and repeating the backup, it is performed normally. Here is the conclusion of the LS team on this file:
-rw-r----- 1 root www-data 144K Jun 25 08:15 /etc/pve/jobs.cfg
Can you also share the /etc/pve/jobs.cfg?
 
Oh, so you have set up an individual backup job for each VM... that will most likely lead to all sorts of locking and timeout issues, as each vzdump job will try to get the global lock. And if it cannot acquire in time, the job will fail. I would recommend to group the backups for VMs to same storage and similar backup times into one job so they are executed in sequential manner without overlap. Further, if necessary you can adapt the lockwait option for vzdump, which will adapt the time a vzdump job will wait before timing out. But that is more of a last resort, restructuring the backup jobs first is recommended.