Ya, weird indeed.
Have you tried in a new profile? (Make sure your original profile does not have any open windows; only the new profile should be open.)
If the new profile also has issues, I would maybe suggest using the Network console in DevTools to see where the 400 is actually coming from; F12, then go to the Network tab, make sure the red dot at the left edge is enabled (to record network logs), and also that the ‘Preserve log’ box is checked. Then try loading the page and pick out the details for the log entry that results in the 400 ‘status’ value. Maybe something will jump out at you as a clue to the problem, or you can share it here and see if we can help spot anything.