500 Can't connect to www.drk-in-karlsruhe.de:443

Hi

I am trying to check the site https://www.drk-in-karlsruhe.de via https and am getting the following error:

500 Can't connect to www.drk-in-karlsruhe.de:443

With http://www.drk-in-karlsruhe.de everything works peachy.

Could you tell me if there is any misconfiguration on my part? The site is accessible through all tested browsers and I even installed w3c-validator with html5 support on my local ubuntu which also could parse the site via https. 

The ssl certificate uses 4096bit rsa and sha256. Might this be the problem?

Am I doing anything wrong, so that w3c-validator cannot connect to the site?

Thanks for your help.

Kind regards

Olivier