Almost all my icons / glyphicons are gone in the preview column of my studio. Started a few Studio's ago (Version 8) and is still happening in Version 9.3. Weird thing is, if I select one without the preview, the icon does show up correctly in the Navigation screen afterwards. What could cause this?
Maybe you should try removing all mendix studio installed, run a system cleaning and make a fresh installation. I’ve never saw this kind of behavior, but it looks like some garbage is causing it to happen.