I had this problem where I couldn't unlock a file from the client side. I decided to go to the sever side which was much simpler.
On SVN Server:
Locate locks
svnadmin lslocks /root/of/repo
(in my case it was var/www/svn/[name of Company])
You can add a specific path to this by svnadmin lslocks /root/of/repo "path/to/file"
Remove lock
svnadmin rmlocks /root/of/repo “path/to/file”
That's it!