The bug seems to be in either a theme or a plugin that you’re using on the site. When I enter the site in safe-mode by going to https://forums.online-go.com/safe-mode I’m able to add a username to a post without any problem. I didn’t try actually saving a post, but I’m fairly sure that will work in safe-mode. If I don’t enter safe-mode, I get exactly the error that you are describing.