Are you using CloudfIare, mod_security, or similar? (Yes / No)
unknown, likely not
Are you sure your provider isn’t using mod_security or Cloudflare or something? That’s a typical cause of unexplained front-end issues.
Please re-check your browser inspector again (like you did in the original report; instructions are in the support template). While reproducing the behavior monitor first the Network tab for anything that fails to load. Then the Console tab for any errors.
I checked my nextcloud.log after setting it to loglevel = 1, and I see “suspicious_login_detected” from app “suspicious_login”. It also says “message”:"Notification was not parsed by any notifier.
The suspicious login app just notifies about suspicious logins; it doesn’t prevent logins.