Here's what I've found for me...I just keep posting "test" in this thread until it goes through, usually it seems to be easier with one or two word posts. Sometimes it takes 10 tries, sometimes 50...but once you are able to post once you are "in" so to speak, and can post again for at least that session. When you close the tab or window or browser or if a certain period of time elapses you are "out" (even if still logged in) and have to try again.
Also, the only browser that seems to work on PC is Firefox for me...on mobile I can post on Chrome as long as it's in Incognito mode. Go figure

It's extremely frustrating though because it's random. I support BTB's suggestion to roll back to a previously, more stable, version of the forum DB. Previously we were having issues with the forum being down sporadically, but these login/timeout posting issues all started in the last week or two.