I checked this server tonight. The problem was caused by the /var/www/imscp/gui/plugins/PhpSwitcher/sql/*
Maybe there was an sql error when BigBen updated the PhpSwitcher, but lot of sql executed before the error. So the next time when he tired the installation again then he has got another error because columns was droped already at the first update. We deleted the sql files until the actual error and retry the install again and again while this folder will be empty. After all sql executed the PhpSwitcher works again.
So if you have similar error then check that sqls and you will found the error.