When I create the post, it gives me that error, signs me out, and removes the user from my user list.
Edit: I can create a post here with that account while on my pc browser.
solved? looks like it works now... I just created another test post with my lemmy.world account without issue. Idk how things are different now.
You can see in the screenshot that your login session appears to have expired. Im not exactly sure what caused that though. If you figure out a consistent way to reproduce it please let me know.
I can’t reproduce it reliably, but it only affects my .world account. Maybe that's a fraction of a clue. Sometimes comments fail to post, then work fine for several attempts, then fail again, never with other accounts. I’ve tried adding/removing the account. This video shows it failing and working across multiple accounts, but no errors appear. (Sorry if this isn't helpful at all.) https://files.catbox.moe/w026ss.mp4
The latest update doesn't have a fix but it adds a little error icon when a comment fails send. If you tap the icon it will show you details of the error. You can give the update a try and see if it uncovers any additional clues.