Symptoms

Backing up a container fails with an error like the following:

INFO: set max number of entries in memory for file-based backups to 1048576
[...]
INFO: Error: upload failed: error at "some/path/inside/the/container"
[...]
ERROR: Backup of VM 101 failed - command 'blah blah blah' failed: exit code 255

Count the number of files in the specified path inside the container. It’s probably over the max number of entries specified in the earlier log line.

Fix

Increase the max number of entries in memory for file-based backups by editing /etc/vzdump.conf on the host. Add a new line and set the value appropriately:

performance: pbs-entries-max=2222222