I am using MacPorts SVN 1.8.9 on OSX Mavericks:
svn --version
svn, version 1.8.9 (r1591380)
compiled May 15 2014, 11:06:03 on x86_64-apple-darwin13.1.0
In the end I had to make the owner of ~/.subversion/auth/
the user I was using. After that the username and password were stored in the auth folder under /Users/userinqu/.subversion/auth/svn.simple . And yes. It is all working now.
For me the problem was that the keychain already had an entry stored for my credentials, but the access rights were not right.
Deleting the entry in the key chain app and then recreating it by using svn fixed the issue.