Please try if this works better on your setup.
It might be that the problem caused that some database records were missing on the slave,
in that case I recommend that you simply copy the ispconfig database over from the master
to the slave before you run the update. This can be easily done by exporting the db on the
master with phpmyadmin or the mysqldump commandline application and then reinsert
the data on the slave. Please make always a backup of the ispconfig database before
you do any modifications, just to be on the safe side