I'm also having similar issue, where I have thread retention set to Forever, and when I keep the thread open in a tab a leave it as is for a while, I get the same screen when trying to reload that tab again.
Often times, I've already forgotten what thread that tab was, but there are some cases where Safari keeps title, and I can later search for that thread that still exists. It's behaving as if the id of the thread has changed?
Thanks for looking into this!