Hmm. I did have a (trivial) html in site-python, and moving it away does indeed make the editor work. But this still doesn't look like the full story, since the module was a few month old. Why didn't it cause any problem before? Reinstalling 1.41 worked fine.
Also, possibly unrelated: I'm now running into
this problem, which was supposedly solved. I'm pretty sure I'm using the latest version of the sources.