During the upgrade of my ISPConfig3 testing-server from Debian Lenny to Squeeze, one problem occurred: MySQL-server-5.1 could not start. This happend, because during the upgrade to MySQL-server5.1, i chose to keep my own /etc/my.cnf file. After some investifation, i found out that that the line "skip-bdb" within this file was the cause of this. So, if you prepair an upgrade from Lenny to Squeeze and your /etc/my.cnf file contains the line "skip-bdb", please comment it out by placing a hash-tag (#) in front of this line. Then your upgrade to MySQL-server5.1 will be succesful! Hope this info will help others too.