[@robin-w Moderator 8 years, 1 month ago]

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

Plugins

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.

Themes

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