Hi Milan,
Please try browsing using Guest profile, navigate to PL-400 page, click button to schedule exam (or use link below), sign-in, etc. In Chrome you access Guest profile by clicking on person icon in upper right corner --> Guest. In Edge click on person icon in upper left corner --> Other profiles --> Browse as guest.
Link to schedule PL-400:
I received same error as you several months ago while attempting to schedule and was able to resolve it by trying multiple times, closing tab, opening new tab, etc., so the above browsing using guest profile should work.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP