Google Referrals dropped - what happened

Hello,

we noticed this month that our Google Referrals dropped by two thirds - that's a lot

Site search @ Google gives me more than 900 000 results:
site:mcseboard.de - Google Search

Using the latest vBSEO and vBSEO SiteMap.
All green on Google Webmaster Tools.

What's going wrong?

Thanks

You have been installed for about 1.5 years, so this is strange.

Have you experienced any downtime recently, or made any significant changes?

Has the # of keywords delivering traffic dropped off, or just traffic to your top terms?
Measuring Your Long Tail - vBulletin SEO Forums

Have you noticed the emergence of any specific competitors that have displaced you in many of the terms you rank for?