Further Details
Thank you for your quick reply. Here are further details.
Plesk Panel 11.0.9 (upgraded from 10.4.4), Microupdate #31, OS Debian 6.0.6, Server: Intel Xeon E3-1245, Quad-Core, 16GB RAM, 3 TB HD, Softraid, enough free disk space (~90% free)
1. Make a backup with plesk...