Error when assigning Activity to a contact
Ah, you have the answer in your question. If you look at your reference to the current code, it should be recognizing that MysqlColumnNameOrAlias instead of generating the error in the default clause of the switch.

So I checked the history of that file, and the MysqlColumnNameOrAlias case was only added mid-last year.

Conclusion - upgrade your Civi to the latest 4.6 (which you should do anyway because of security reasons).