I had an issue with the profile pages 404’ing.
In my case it was down to the ‘Forum Root’ i had set in the forum settings. I set a ‘Forum Root’ with a preceeding / like this ‘/forum’.
I then ticked the ‘Prefix all forum content with the Forum Root slug (Recommended)’ box.
This resulted in rewrite rules for the user pages like this
[/forum/users/([^/]+)/?$] => index.php?bbp_user=$matches[1]
which did not correctly match. I removed the proceeding slash from my ‘Forum Root’ so it looked like this ‘forum’ and then the rewrite rules that were created looked like this:
[forum/users/([^/]+)/?$] => index.php?bbp_user=$matches[1]
(no proceeding slash) and it worked.
Maybe the ‘Forum Root’ should have any proceeding slash removed by default to account for this scenario?