Hi Corey
first of all, thanks for your reply.
I have tried what your suggested, but the error is still the same. I did also double check the permissions, files names, path and config-file and it looks all OK.
In the meantime I did verify the certificates, too, and I was able to connect to the Apple Push Server via openssl successfully (no errors):
openssl s_client -connect gateway.sandbox.push.apple.com:2195 -cert MY.cer -key MY.pem -CApath /etc/ssl/certs
... Verify return code: 0 (ok)