Like Carlosan suggested, try updating your graphic card drivers, because visual glitches in the UI can sometimes be attributed to a card driver. The other thing I would suggest, is to temporarily rename the (My Documents/3DCoat) directory (something like 3DCoatTEMP). This will force 3DCoat to create a new directory from scratch, once you restart the application again. It is possible an XML file got corrupted, somehow, and could be the culprit.
If that fixes the issue, you can copy and paste the most important folders from the old one, into the new one.