The most likely cause for this error is a corrupted psa database. I suggest to restore the latest psa database backup. Either - if your psa dump is current - by using the latest nightly dump:
# cd /var/lib/psa/dumps
# cat mysql.daily.dump.1.gz | sed -n '/-- Current Database: `psa`/,/-- Current Database:*/p' | MYSQL_PWD=`cat /etc/psa/.psa.shadow` mysql -uadmin
or using a previous dump from /var/lib/psa/dumps (you can see them ordered by date). Here is another way to do it: