The problem is, that www.synology-forum.de sends broken HTTP responses when the browser supports gzip content encoding. It sends a Content-Length header value of the uncompressed data where the size of the compressed data must be send. However as there are more such broken servers on the internet (http://forum.avast.com/index.php?topic=100557.0), we will see if we can find a workaround for them.
Well, there is no need to disable gzip content encoding, it must only be done properly according to the HTTP standard. But I suppose this is just a temporary solution until they fix their server configuration/content generation logic.