possibly a conflict, or maybe your site has been hacked.
is this just one username or all ?
It could be a theme or plugin issue
Deactivate all but bbpress and see if this fixes. if it does, re-enable one at a time to see which is causing the error.
If plugins don’t pinpoint the problem, as a test switch to a default theme such as twentyfifteen, and see if this fixes.
Then come back
Thank you for your reply Robin. The issue turned out to be a conflict with the themes customization area for a “custom 404 page” instead of using the default 404 page. When I turn this off, and use the default 404 page, the issue resolved. The theme developer is working on a fix on their end. :o)
great – glad you have found the issue – and thanks for reporting back !