You can't search users' posts/threads while you're logged out. I imagine this was intentional, so wouldn't it be better to display a message to guests asking them to register and/or log in to use this function? Now, and for the past 2 years I remember, it shows the "You broke the internet' message with suggestions that won't work for this.

This should be fixed now, thanks for the heads up.

 

Thanks for looking into this.