Hello,
we experienced a similiar issue.
Since PL 7 there is a bugfix included which removes compatiblity mode for IE7 by removing some registry keys which handles compatibility mode of the internet explorer control in general. (see: http://msdn.microsoft.com/en-us/library/ee330730(v=vs.85).aspx)
In our case, after this fix, Java Applets were no longer working.
The difference for IE11 now is, that it identifies itselfs no longer as MSIE but as Trident-Engine.
The solution for us was to update to newest deployJava.js which added support for identifing Trident as MSIE.
So perhaps http://maps.adac.de has also to add support for Trident-Engine in browser detection, which is unfortunately not in your hand.
But you could add the registry key, which was removed since PL7 again by your own.
kind regards,
Michael