When I want to search by user name, I left title search keyword as blank, only filled user name, then search didn’t worked.
The only way I know to look for their content is by clicking on the member’s name on one of their posts, then click either “Find all content by [name]” or “Find all threads by [name]”.
I think it’s because it’s only meant to be used in conjunction with keywords and not just a user search.