It seems to be different for everybody and appears to go in cycles. Two weeks ago I was able to sign in normally and started to keep track of the days. A couple of days ago that failed and I had to resort to a work-around. This is using Firefox with no cookie or data manipulation. Once this cycle has run its course and it's back to normal, I'll post the timeline and the tricks I used.
Okay, I've gone through a cycle with Firefox without resorting to deleting cookies or site data. Here is how it's gone:
Monday Aug 23 Community Forums sign-in works 4:55 PM 9/3/2021 Loop started. Went to PayGo balance & scrolled down to CF, which worked. 11:58 AM 9/11/2021 Back to normal.
This time I didn't have to resort to using a private window before it went back to normal. The whole cycle was 19 days and it's been normal now for 2 days. I'll report back when signing in goes into another loop. Going by what others see, it's not the same for everyone. Still a mystery.