Recently I had few problems with a svn repository that is shared between multiple ssh users. I followed the instructions in the svn book and to solve the problem once for all I recreated the repo from scratch. Briefly:
svnadmin dump SVN > /tmp/dump
mv SVN SVN-old
svnadmin create SVN
chmod -R g+rw SVN
find SVN -type d -exec chmod g+s {} \;
chown -R root.cvs SVN
svnadmin load SVN < /tmp/dump
hopefully this is gonna work. hopefully, otherwise I guess I missing something that is very basic !