"Visit Over Time"-chart does not update

Hello there

I'm having troubles with the visits over time chart. Since I moved piwik to a linux-cluster the chart does not update anymore. Actually, it does log the visits you can see that in in the widget visitors in real-time.

I updated Piwik to the version 1.12 but this does not help neither.

I would be glad for any help and ideas.
does anyone know which script does generate the visits over time chart?

Thx a lot.