Corrupted page formatting while using https

[gdur]

I just have set up a test site which I have to approach via https while not being in the office (not in the DMZ where I can use just http). Some strange page formatting behaviour appears which I'm not able to resolve. Depending on the content, a page will format correctly or not. Sometimes switching on/off the news module helps but certainly not for all pages. I cannot find any consistency.
This is only happening while using https. With http everything works as expected.
config.php has been altered as documented in the FAQ, http or https is stored in a variable, so this should not cause the problem.
This does not only happen with my own defined content but also with some of the content which comes with the install.
Looking at the html source of the page in the browser it is obvious the code is abrubted randomly.
Does anyone has an idea?

here is the url to visualize my problem...
https://mail2.duran-audio.com/cms/

[Dr.CSS]

I get a "Don't trust this site" warning when I try to access it...

[gdur]

Yeah, I know. This site has a self generated certificate. No worries it's save to go there....