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
).