Server certificates are not properly validated by subversion/openssl
A friend (who was too lazy to write the answer here) hinted me that the root certificates in Mac OSX are stored in the keychain and suggested two different ways around the problem.

In order to make openssl library (used by svn) to locate them, you need to manually export them and store them in /System/Library/OpenSSL/cert.pm. The process is described step-by-step in the following blog post: http://jw35.blogspot.gr/2011/02/root-certificates-for-macos-openssl.html

If you are using homebrew, an alternative solution is to install curl-ca-bundle package and point openssl to the location of the certificates with the use of an environment variable.

brew install curl-ca-bundle
echo "export SSL_CERT_FILE=$(brew --prefix)/opt/curl-ca-bundle/share/ca-bundle.crt" &gt;&gt; ~/.bashrc