I edited it much after the first 5 minutes.
Furthermore, I tried again to edit choices in the initial problematic post (made 2 days ago) and I can edit 5 out of the 6 polls that exist in the post … it’s just one of them that kicks in the 500 error. There is something about that one! It is
[poll name=multipart]
* <choice 1>
* <choice 2>
* <choice 3>
[/poll]
and if I try to add even a single character, I get the 500 error. Editing any of the other 5 polls in the same post works just fine.
If the error was intentional, then I’m thinking it wouldn’t be a 500 error. This all sounds like a bug to me.