This error was appearing for me weeks ago, but under different circumstances
I was seeing that error when trying to view a thread in the buyer/seller area.
I thought a mod or admin nuked a thread created by somebody else who gave me good feedback, which was followed by other positive feedback from others who bought my excessive TiVo-related items (cleaning house, not running a business here).
I posted in one of the actual threads on something I sold that I wished I had printed a copy. The positive comments were just so nice, as opposed to how things can be in the on-topic threads.
Then, the thread magically came back. How did I know the url to the thread that went missing? I had posted a link to the feedback thread within the threads for things I still hadn't sold.
During the time it was "missing" there was no trace it ever existed, and I experienced no other issues with anything (that I noticed). I just couldn't see it in my subscribed threads, listed in the buyer/seller area, or find it via search. I just had that url, and it gave that error.
So, something was going on before the crop of things that started threads about this error yesterday. I thought maybe this info might prove helpful to the admin(s).