Symptoms
Plesk backup finishes with the following warning message:
Warning
Subscription "example.com"
Not all the data from /path/example.com was backed up successfully: lscache/.cm.log: Cannot open: Permission denied
Cause
Incorrect permissions of LiteSpeed web-server cache files.
Resolution
LiteSpeed web-server cache files do not need to be backed up. Thus, such warnings can be safely ignored.
In case these warnings need to be avoided, exclude mentioned lscache/.cm.log file/files from backup creation according to the following Plesk guide:
How to exclude specific files/directories from a backup in Plesk?