I used not my main user. However, only the main user can get shell access necessary for svn via ssh.
I used a wrong path.
In order to get things going set up subversion following the usual directives in the knowledge base and then access your repository like this:
(OS X terminal here)
% svn co svn+ssh://firstname.lastname@example.org/home/mainuser/svn/repositoryname/
This should do the trick.
Remember not to use ssh and http(s) at the same time. It's either or.