I see visitors in real-time but recent vistors graph has a flatline

Since 21 of april my Recent Visitors is flatlining on me. I can see there are visitors in the Real Time box though. I have about 20 sites and they all show this behaviour.
If i make reports i see a hole in the stats too.

I am running the latest version of Piwik (2.12.1) on Windows Server 2008R2, IIS 7.5.
Anyone else experiencing this problem? Or maybe someone can point me in he right direction for a solution?

Kind regards,