Once in a while, I ran into this error with subversion:

svn: This client is too old to work with working copy ...

There are a couple of senarios:

1. The subversion is indeed too old, then we just upgrade it.

2. We temporarily tried a newer version of subversion. And the trial upgraded the format of the svn metadata.

3. We copied (rsync) the entire directory from another server that has a higher subversion.

For the cases of 2 and 3, we just need to downgrade the format of svn metadata. http://subversion.apache.org/faq.html#working-copy-format-change has a script to do it.