Suddenly panel login is no longer possible - "Cannot GET /api/auth/login"

Urgent question:
Overnight, I suddenly couldn’t log in to one of my Kirby installations anymore.

The error message is a blank page with the words
Cannot GET /api/auth/login

When I enable debug mode, nothing changes, and there’s no additional information.
I have no idea what could be causing this, because nobody has made any changes.
Could a change to the server (and if so, which one?) be causing this issue?
Thanks for your help!
Best regards, Matthias

The endpoint requires a POST request, not a GET request, so maybe something has changed on the server indeed