When I tried to delete a new user who’d posted a load of foreign-language spam I got this message. I’m on 2.9.0beta14. It was the same whichever of the two “delete” options I chose.
Edit: Just noticed that the user did get deleted (“Deleted via review queue”) so maybe I just clicked the button too many times.
Sorry for the delay, I meant to reply to this one at the time, but for some reason did not get round to posting the draft…
I also see this a fair bit when deleting spam users. Everything works as expected, but you do get the “An error occurred…” modal pop up, which can be misleading.
To replicate:
Sign up test user
Paste a chunk of Lorem Ipsum in for their first post to trigger 'New user typed their first post suspiciously fast, suspected bot or spammer behavior.`
When reviewing, select ‘delete user’
Receive error modal (though everything has worked and the user has been deleted)