If you have problem to use subversion under redmine, but svn command itself works ok, the problem might be in incorrect home directory configured for user which is running redmine (can be apache user, fcgi user id…etc). Incorrect here means home directory points to a file instead of directory (ie. /dev/null) . One can reproduce this with setting HOME to point to file.
$ HOME=/dev/null svn --version svn: Can't open file '/dev/null/.subversion/servers': Not a directory $ HOME=/dev/null svn --version --quiet svn: Can't open file '/dev/null/.subversion/servers': Not a directory
Solution is pretty simple, just change user home directory configuration (via usermod) or set somehow $HOME variable of the redmine execution environment to point to some directory (ie. HOME=/var/empty).
This misbehavior has been reported as subversion defect.