Google is no longer indexing hidden div tags. Spammers used hidden divs to hide content, but also many non-spammers and even non-SEOs have used them for design purposes. The theory is that the Jagger update now hurt any site using hidden divs.
...also wer es noch nicht hat, CSS extern auslagern und von robots.txt ausperren...It may be because the CSS is off the page. Supposedly, if you do not have it on page, then this wont be an issue. Maybe that is why some sites have been affected. Should they be? Most should not. But that is a Google update for you.
So what can you do now? Move that CSS off the page, to an external style sheet. Then maybe, block the robots.txt from those files.