[git] Git - Won't add files?

I had a similar issue.

The problem was, on Windows, in the index, the file was added in a case different from what was in the unstaged area. For example, in the index, the file was name xx.txt and in the unstaged area, the file was names Xx.txt.

Removed the file with the incorrect case from the index (xx.txt). Then, I have been able to add the file with the correct case (Xx.txt).