Playing with editing the Profile and tried changing my nickname to KarenS and then to Karen. Both of those times it said the nickname was already taken. Then tried KarenABC and that worked fine. Went over to the backend and looked at users, and I was the only Karen so seems odd that it’s giving a message that it’s already taken. Are the Communities users stored in a different spot? Guessing there is no bug since I can change it to KarenABC.
The user database is complicated. It is shared with the Core, but users are only migrated over when this visit this sub-site. Which only the alpha testers have. So you likely saw a collision with a user that exists on the main site, but not this sub-site yet.
