Backup failed

[kae14k]

if we make a backup for a domain with a mysql database, we get following error.

Backup process has failed for domain 'test.com'
Input string was not in a correct format.

---------------------- Debug Info -------------------------------
0: C:\Program Files\SWsoft\Plesk\admin\plib\common_func.php3:152
psaerror(string 'Backup process has failed for domain 'test.com'
Input string was not in a correct format.
')
1: C:\Program Files\SWsoft\Plesk\admin\htdocs\domains\backup\download.php:154

A backup for a domain without a mysql database works fine.
We have installed Plesk 8.1.1 and mysql 4.1.