White spaces in username give 404 on author page

Spaces are not allowed in URLs, so WordPress uses the user_nicename for those links. The user nice_name is “A string that contains a URL-friendly name for the user. The default is the user’s username.”. That is a transformed user name such that it is all lowercase with spaces replaced by dashes. I did not look up what other transformations are probably occurring.

Whatever is creating those links is doing it wrong. Without seeing the code that creates the links, it is hard to say more.