in file server/lib/classes/backup.inc.php
The restoreBackupWebFiles method gets passed a $filename
In one branche it is getting concatenated into $full_filename
This fixes the variable name from $full_filename to $filename on line 451
Line 451 is outside the branch described above

Fround by: PHP Deprecated:  is_dir(): Passing null to parameter #1 ($filename) of type string is deprecated in /usr/local/ispconfig/server/lib/classes/backup.inc.php on line 1311

This reverts commit 764f10b4