Rules won't convert to advanced and the editor won't say why

[SpurrBeeversWoodBuxton]

When I select "convert to advanced rules" in one of my files - after the dialogue box asking if I'm sure and explaining the process - the editor brings up another dialogue box titled "Convert to advanced rules" with no text in it and an "ok" button. Clicking the button takes you back to where you were. This is like when you try to convert something that's not verified, right? (Except that in that case it would tell you the rules aren't verified and in this case the rules are verified, so...)

Anyone else had this? Is there a workaround? Is it a bug that I should report?

The file was made by "adding leagues and cups to existing nation" (Scotland) if that matters.