I have encountered something in the demo site, for example in this course: https://school.moodledemo.net/course/view.php?id=69 , when quickly and repeatedly clicking 'Collapse all'/'Expand all', I get shown an popup saying 'undefined'. When looking in the Developer Options in Firefox, I see that the AJAX Request responded with an 503. Is this some kind of DOS protection, or actually a bug? If it's a bug, could someone maybe reproduce this and see if there are any useful information in the server logs, so that one could open a MDL tracker issue and properly tackle the bug?

We have a similar problem in our (server hosted) moodle 4.0 test instance (resets the users session in our case, which than causes JS to redirect to login), but no idea or information where and why the problem occurs, and I can't reproduce it in a local (debuggable) instance, so I was hoping that maybe the 503 on the demo site is a sign of something similar happening, but aborting instead of giving an erroneous response.

Thank you
Justus