I was looking for the error in the wrong place... I have the "Change Content Owner" addon running, which gives admins the option to change the author during editing. We needed this to manually transfer some posts from an older forum.
Now, after an update, LastPass started filling in fields that were not filled in before. So then my email address was entered into the field for the new owner. (I actually turned off autofill and use the Lastpass icons). Anyway, xenforo then naturally did not recognise the email address as a valid user and therefore generated this error message.