When the user goes into their account settings to change their nickname/profile URL and clicks "save", the page refreshes but the old nickname just repopulates the field and nothing changes.
Admin is not able to change the nickname for this user either. Or for my own admin account, for that matter.
BUT other users CAN change their nickname, and admin can change their nickname
So some user nicknames just refuse to change.
Is this a known bug? Can I change their nickname/profile URL on the backend?
You have several JS and other errors in your pages. Maybe that is the cause, but in any case many things do not work right.
Definitely have JS errors, but the fact that it ALWAYS works with some users and NEVER works for others leads me to think its a bug with the plugin
And if it isn't, could I change the nickname/URL on the backend to just go around these issues?
You could also enable wpForo debug or Wordpress debug log and check.
Also could be WP Rocket you use ?