no ssl certificate data found - copy ssl to postfix

I am using the Virtualmin 6.06-2 and have successful installed a certificate on the domain. It works well on the Webserver.
When I try to compy this certificate to Postfix via the web interface or command line I get following error message:
no ssl certificate data found
The same error is shown when I try to download a pem file of my certificate.
What can cause this issue?