Hey guys,
after several hours of error messages I was able to submit my application. A friend of mine gave me the hint to use safari instead of google chrome, internet explorer and mozilla firefox. Safari seems to be known as a browser with less errors.
This problem with the webpage is very common says the visa helpline desk...
Moreover Alimoh is right: Keep reloading the webpage, even if you get messages like "error 503 - page unavailable" or "This on-line services are currently down– system generated error code: XXXXXXXX”. Sometimes you´ll get some answers, you just answered, but keep trying and answering, it´ll work in the end.
Be patient!