The path to the xdelta library that's entered into admin > course > incremental backups field 'xdelta path' should allow relative paths to be entered.

Allowing absolute should still be possible as this will be useful on the server however for the Offline Moodle client where the drive letter or entire path may change at any time the path should be relative to the moodle data directory. Real path should then be used to get the full path and \ turned to / as mkdir and rmdir has an issue with these.