You might have to access the server and from the command line change into the hidden .git directory then del the index.lock file if it exist. Check how your client app is accessing (user) the .git directory ... as the owner of all files/folders contained therein? ... ie, who owns the files and what permissions does that user have on files.
Google search also mentioned xcode php extension might be involved. Unload that extension and see what happens .... you can always reload it.
'spirit of sharing', Ken