Card Validator and SSL: internal error


We are trying to validate a twitter card that is accessible through a SSL secured connection, but we always get the following error: Fetching the page failed due to an internal error. We also made sure that the page does not contain any mixed content, so everything is SSL secured. The same URL without SSL works without problems.
We also tested it with tweets, there the card is displayed when using the NON-SSL url, when using the SSL url, the card is not displayed.

Any ideas?

Edit: We are using a RapidSSL cert and are also correctly sending the intermediate ca.