I’m new here, and new to HG so maybe wait for someone more experienced to answer…
That said, I’d try editing the groups.xml file.
It should be in your executable directory. Backup first is always a good idea. If you don’t know xml I can help a bit, but any text editor can do it. SublimeText2 will preserve the formatting and make it look pretty.
I had the same problem some time ago.
This happens when inserting a new module, it is inadvertently clicked on the descriptive bar that separates the different modules.
In this case, a line of code with wrong syntax is incorrectly inserted into the groups.xml file. (Small Bug).
Just delete this incorrect line in the xml file which is not difficult to recognize.