I was getting this error and it seemed my working copy was corrupt. A sure fire way to fix this, if you can afford to, is to remove or move the folder with the error and update to get a clean copy. But just now I had this with a folder of 300MB of images.
The following quick hack worked though :
- copy some other file like .svn/text-base/somefile.ext.svn-base in the same folder to .svn/text-base/somefile.ext.svn-base
- update to a version of the repo before somefile.ext existed (or maybe changed - not sure) like this "svn update -r XXX"
- update to the current version "svn update"
The above should get past the error by having a correct-looking file in the expected place. We then cause that file to be replaced with its correct content by causing svn to overwrite it