This only happens in Skirmish mode. What is the issue, and how do I fix it?

Mods, most likely. IF you have any, remove them one at a time until the issue fixes itself.
If you don't have mods, make sure your resolution is set to at least 640x480, for this is a problem with the names of buyable items being too long for the game to render properly. If a PresetName contains 11 nonspace characters in a row the game will crash (though if you set your resolution higher you can increase that number).