i got a response from siteground support.
it seems to be definitly a template issue. i need a solution urgent please
Hello Dennis,
I have checked and noticed that the error you are receiving is application related due to the fact that the provided 404 errors which are generated from some of the articles are custom application side 404 pages and not a default server one.
Upon a closer investigation of the issue I was able to find out that the issues are mainly with the content from the K2 plugin.
I was also able to see that the issue is caused by the template which you are using.
In order to verify this I used the following link when I had used your current theme and when I was using a default Joomla! template:
http://www.xxx.org/item/303-heilpflanze-loewenzahnWhen I was using the default Joomla! template protostar the item was displayed without any issues, although when I am using your current template you are being redirected to the main page.
I would suggest that you contact your developer regarding the issue or the themes developer.
Do not hesitate to contact us should you experience any difficulties.
Best Regards,
Dimitar Geshev
Technical Support Team