I just spent quite a few hours trying to figure out why I got the error message in the title of this post.
There were many suspects: The reverse proxy, threaded perl, bug in the authentication script, bug with SVN…
I ruled out these one by one.
In the end, I added the directive “SVNPathAuthz off” to the SVN locations configuration in apache. And the problem was solved. One line of config, five hours debugging. Wow.