I've seen a good amount of articles dealing with older svn versions hitting this problem, but my situation is as follows.
I am using Subversion 1.6.6
I have 'versions', a mediocre mac svn gui, which uses the command line svn tools (version 1.6.6)! My repo (svnserve v1.6.6) is located on a max box on my local network, I'm accessing it via svn+ssh.
The box that hosts the repo (also up to date on svn v1.6.6) can connect via file:// but not via ssh://
The repo was created with versions on the host and worked reliably for some time. I'm not sure what the tipping point was, but it may have been me creating a second user specifically for ssh connections. Now the repo cannot be accessed from either user via ssh.
here is the command line attempt..
user$ svn up svn: Expected FS format '2'; found format '3' user$ svn --version svn, version 1.6.6 (r40053) compiled Oct 22 2009, 14:13:09