Since this just started up again, and there has not been any changes with the chat/forum interface other than nuking the banner, I would say Safari bug from the number of occurrences I am seeing.
Safari must have changed something in how it detects the 'userid' field.
If the user is already seeing the chat input box, then the user is already logged in so I guess Safari, if that is the common browser, is missing that.
I could put a banner back in, 'This banner exists as a security fix for poorly written Apple products' or something like that.