I was just given this screen, every time I tried to change
to the next or previous thread, or anywhere.

Yup. I've been getting that a lot lately. In my case, it will suddenly pipe up with
We were unable to process your request. Reason
One of the required fields was not filled in.

It seems to happen if I'm here for an hour or more, and the only way I can get back in is to shut down my browser, completely delete all my cookies, and clear my internet history, then login afresh. I have a handy batch file that empties all the relevant folders for me, but that requires rebooting to DOS, so I normally do that when the message comes up - a real pain in the ask anybody.