Hi Georg,
I have had the same problem. It is because of the latest update by Google around 'over-optimisation'.
Don't worry though, all is not lost! There are a few things that you need to change:
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Make sure that your webpage titles are not over-optimised (i.e. 'Free Shoes, New Shoes, Sports Shoes - Shoe Company'). Make sure that they read properly and don't look spammy. Try not to repeat any of the keywords more than once, if at all.
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Check that all of the links on the page are not suspicious. For example, having loads of anchor text linking to the same page.
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See what your keyword density is on your webpage and see if you have maybe be keyword stuffing (Google is being a lot harsher and stricter on this now).
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Go through your img alt attributes and check they are all relevant and not just blatanet attempts to fit in long-tail keywords.
Once you have done this, resubmit the pages to the Google index and hopefully, after a few weeks, you should start regaining some ground. Unfortunately its a bit of a waiting game now, but make sure you do everything you can to show to Google that you are not over-optimising and you should be OK.
Also, take a look at Rand's recent whiteboard Friday which goes over this.
Good luck!
Matt.