Error message on trying to log in to forum

I'm still frequently getting this error message when I try to log in here. 

I can get back to being able to log in by clearing website data for this site in my browser (Safari, using MacOS 11 Big Sur - but I first encountered the issue before I had updated to Big Sur). 

Why does this keep happening? It is getting very annoying to have to keep clearing the cache every day.

Ian