Backups in Plesk - including scheduled backups - can not be created with:
Error: The backup process failed: can't concat str to bytes
Plesk Obsidian has been recently updated to version 18.0.47.
#PPPM-13733 "Creation of a domain backup no longer fails when the “Exclude log files” or “Exclude specific files from the backup” options are enabled."
- Plesk Obsidian 29 September 2022 (Linux)
If update is not possible for some reason you may try the following
As a workaround, create backups including all files.