Hey, @laurence, I’ve been seeing the 400 error in the app editor as well. My issue is related to plugin preview rendering. I just submitted a bug report yesterday.
I’m developing a plugin and started seeing that error when the size of the function for rendering the preview grew beyond a certain point. The issue only occurs in the app editor when my plugin renders the preview. I don’t know if your issue is related, but it looks similar.
@eve just replied to my bug report, and I sent additional info. I also just made this comment on the original preview rendering thread.
I’m eager to get this resolved, as I’m now convinced it’s a Bubble issue and not a problem with my code!