Settings : site contact username

I’m having a small problem where I can’t set the “site contact username”. I’m getting the following error

There’s no user with that username.

The status code return

422 No Reason Phrase

The user name specified in the field is @frez which exists

I can set “system” as valid setting and the return code is

200 ok

Not sure what to do here.

The user has to be staff (i.e. moderator or admin) to be the site contact, I think.

It’s the main admin account.

Ah apologies, forgot that didn’t automatically change trust level. Maybe change trust level to 4 (on the offchance) and check if capitalization is significant (it isn’t elsewhere, but hey)?

Otherwise I’m out, sorry, hopefully someone else has an idea.

edit: Actually definitely check the trust level - at least for non-admin accounts, you can’t send private messages without being at least TL1. Maybe that’s causing the issue here.

Doesn’t work either. But never mind.
I’ll just use “system” that will be fine.