I also just tried it again without the .git at the end, and it still works on my end.
So yes looks like Moin may be on the right track with her advice. Have you updated your site to latest? Do you have another component already installed that is conflicting?
The error handling is fairly attrocious here - would be nice if a better error message could be shown! 
Can you check if more/better info about what is going wrong is appearing in the javascript console?