SSL Proxy problem: Part of the URL gets lost

Hi,
I'm trying to use ssl encryption for my site which is hosted on hosteurope.de; it's not a own ssl certificate, but a ssl proxy that I use.

The ssl URL is https://ssl.webpack.de/www.mydomain.com

Surfing around works as it should, but when I try to create content and klick Submit, the URL is

https://ssl.webpack.de/node/101/edit

instead of

https://ssl.webpack.de/www.mydomain.com/node/101/edit

and I get the error message "404 Not Found".

Without ssl, everything is ok.

Is this a known limitation of Drupal? Does anybody have a hint?

Any help is appreciated.