Alright, so the issue is this: I go to a thread, I can post on it normally. Then when I come back a day or so later, it starts asking me for "form resubmission" to view it. And then it does this:

I already lost access to one thread this way, wrote a message to someone about it. No answer. Now I got it on another thread too. Its getting really frustrating. Please, someone, help me.

Eta: forgot to delete my "pinned" top bar from the screenshot. Meh.
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I've flagged this on our side with the forum provider. I'll let you know when I get more information to share
Oh thank you so much! I had this thread as a pinned tab before it happened, in case that is important. OwO (I also may have had a VPN active, on accident.) >.>
That's unrelated, this error is apparently tied to draft messages that haven't been sent being formatted incorrectly, which breaks the page for your account.
Our forum host is investigating now and are trying to clear the issue on their side.
But oh, that is super helpful to know already @EA_Cade thank you! Yeah... saving drafts (and otherwise leaving a post hanging to be finished later) does happen a lot on my side, at least where storytelling posts are involved. ;D
While this is being looked into on the forum host side we have a temporary fix should you run into this in the meantime:
As a workaround, they recommended that you access your drafts page and then press the X on the right hand side that appears when you mouse over the draft for the respective thread you can't access.