We're still aware this is an issue for some, but unfortunately it's still unclear as to the root cause. Browser session time is set at 24 hours, but some browsers are logging people out before this. Refreshing should log you back in.
As annoying as it is, I would highly recommend saving all story drafts offline for now and submitting when ready, and if you're writing a long forum post, do a quick copy of the text before you hit submit just in case it dumps you out and you lose it.
Absolutely accept this isn't ideal, but we will update with any new info. Thanks.