Everywhere I go on google I get the same answer to this problem but it is the wrong answer for me. Originally I installed Subversion 1.5.6 for windows (i run Windows Server 2008
It turns out my server version of SVN was 1.5.6, but the TortoiseSVN client I had installed on the server was version 1.6.3. When I would checkout my wwwroot working copy (or update it using the right click options TortoiseSVN gave me) and updated that working copy's version, the SVN server couldn't update it because it was a lower version.
I downloaded TortoiseSVN 1.5.6 from file hippo and re-checked out the working copy on the server and it all worked great!