It's not a theme related issue. Themes do not make calls to the database for anything. This is defiantely a new block or module that you must have added around the same time you switched themes.

The problem is the offending code is trying to make a call to a table called 'forum._users'. If you have software that will allow you to search multiple files for a text string use that and run it against your entire nuke directory and try to find which file has the string 'forum._users' in it.

If you do not have such software. Just start disabling blocks/modules until you see that line go away. That way you can figure out who the culprit is. Then go into the main .php file for the offending block/module do a search for 'forum._users' and change it to 'nuke_users' then reactivate the block and see how it looks!

Good luck!