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.