Hi all, I think I finally solved this problem! It seems like the issue occurs when somewhere in the MapTool launcher directory is an ampersand (&). So if, like me, you have been keeping your MapTool launcher in your "D&D" folder, try changing it to "DND" instead and the problem should be solved! I'm not sure if this is something that can be fixed in the next version of Maptool, but in any case the workaround seems easy enough. I hope this helps!
I think this can happen if the ampersand is anywhere in the MAPTOOL_DIRECTORY path... not just the launcher folder.
I've noted that earlier users that had this problem had these settings:
User Dir.: D:\My Documents\D&D Materials\Maptool-1.3.b90\MTB90-TEST
MapTool launcher in your "D&D" folder