So the most recent major release of the Subversion source control software used for some KDE modules is version 1.7, released waaay back in October 2011.
It turns out that 1.7 uses a different on-disk format (which centralizes all the metadata to a single .svn directory among many other things). Also, that on-disk format doesn’t get automatically upgraded, but instead you must run a svn upgrade command manually (but just once).
I had somehow managed to avoid 1.7 by accident so I was pretty shocked to see a kdesrc-build bug get reported about this — it was news to me!
Now that I’ve had time to look at it svn 1.7 should work just fine, kdesrc-build will try to run the required command if svn says it’s necessary. If an error occurs and you don’t have local patches you should just delete the affected source directory and let kdesrc-build check it out again. The updated kdesrc-build is only in git-master for now though.
But be careful, there’s no way to run svn diff with svn-1.7 against an older working copy!