Vassal 3.4.6 freezing when trying to save edits to mdx

I just installed Vassal 3.4.6. I’ve been editing some extensions to a module, but when I try to save the changes Vassal hangs. (I had the same thing happen with extensions for 3 different modules.) I can still save changes to main modules.

I’m not sure how to proceed.

What’s in the errorLog when that happens?