We use svn, and I use git-svn to maintain sanity. At one point, our svn server decided to return a 403 for a certain folder. It happened to everybody, not just me.
A better way to do this now is git svn reset -r where rev is the revision before where you want to start refetching. After that's done, just run git svn fetch and it will rebuild its index and refetch the revisions after rev.