Error 1054! Unknown column 'a.modified_date' in 'o - by: Roger Keller

Hi,
I'm using helpdesk in the latest version. All good so far apart from the menu link that should show a list of the tickets. It produces the following SQL error:

Error
1054!
Unknown column 'a.modified_date' in 'order clause....

I can't paste the complete error code here, because your system tells me that I try to inject some SQL.

What can I do?
Thanks
Roger