Anyone got any ideas?
Most likely this is from trying to accept an invite from a browser session that is logged into Plex.
In the browser you are using to open the invite go to http://app.plex.tv/ and in the upper right corner click the down arrow and select “sign out” then click on the invite again.
if above doesn’t work I responded in your other thread https://forums.plex.tv/discussion/288984/error-when-invited-guest-clicks-on-link-in-invite-email#latest
