It appeared to be an "extended character" bug. I got help from a magic wand and voila it works now. Thank you Isaac I am ever so grateful! 🙏
I don't know what that means without some specific information.
NOTE: This was absolutely unrelated to UBB.threads 7.7.x.
Morgan had an random extended character inserted to his USERS permissions. It has probably been there since a very early version of the software or from an upgrade made from a version of 15+ years ago.
What I did to solve his issue was:
With temporary access granted from Morgan: 1/ I went to his: Control Panel > Groups 2/ Clicked [Site] permission for USERS group 3/ Then I clicked the bottom button, "Update This Group's Permissions". The forum list started displaying correctly again. I did this for each one of his groups. I did not changed anything else.
What this did was just rebuild any missing or any using bad (such as spaces or extended characters) user group permission settings.