Sorry, have no idea. Can you describe it a bit more? You try to start variant editor?
I`m using Trylobot`s editor and it has no issues with the Mule.
Literally all I know is that there are two mule standard variant entries under my codex, and if I click on the second one it crashes
If I attempt to use the variant editor in dev mode it crashes
If i attempt to load that variant it crashes
If i go to save during a game and it attempts to bring up that variant image next to the various options it crashes
I can only find one of the mule variant's in data files
The other has a weapon called heatseeker that cannot be found and immediately causes a crash
I tried reinstalling both ironclad and Starfarer still having issues