It has been this way since I tried using the search bar around the 1470 release. Queries through the search bar works fine after switching the filelist view to "Show details" (all text). The error comes back when switching to the view to "Show Art with Details." I even tried optimizing my db (complete), and the error still shows up with "Show Art with Details."TControl.InvalidateControl : can't be called outside of main thread ! (M:\Sources\MediaMonkey403MM\MP3Datab\Controls.pas, line 6399).
Here are the fields I ticked off for the search bar to seek (nothing exotic):

Commentary: The filelist view "Show Art with Details" seems quite buggy in MM4 compared to MM3 (see my sig). Hopefully these bugs can ironed out eventually.
Note: I was about to post about a regression in 1471 with "Find More of the Same > Explorer," but it seems to be fixed in 1472.
EDIT: Using Mediamonkey 4.0.3.1472 debug (full install)
OS: Win 7 x64