Comment 1 for bug 1438960

The little exclamation mark is trying to tell you something - it has a tooltip.

The actual reason, you could probably dig up, has to do with the fact that some OS's linux can interact with are not case-sensitive, and may not realize you are, in fact, trying to connect with a username of jOeuSEr and not joeuser. It's not an issue within Linux at, all, but a policy decision not to allow it that we inherit (and more or less agree with) from Ubuntu.

Btw, I can't speak for Mate's UI in this, just Cinnamon's.