Red X appears sometimes in VSTO Add-In

I have a VSTO Word Add-In. It generally works great.

However, there is one PC that, sometimes, instead of the usual user controls it displays a big red X.

The PC is a Windows 7, 4GB Ram, and the office version is 2010.

I know that generally the red X is displayed when there is an error in the Paint event, however I don't have any implementation of this event in any of my controls.

I've checked everything I can think of, but I couldn't find the cause of this. Any ideas what to check?