No Graph on Dashboard anymore, every other graph shows right (2 replies)

Hello together,

I've recently changed my webserver from apache to nginx and I'm facing one strange error. The smalls graphics on the dashboard does not show anymore (see Screenshot attached). Every other Graphic, also the small ones, are working perfectly fine. No error in the log, no error in chrome development tools.

I've already emptied the cache of Chrome and changed the browser from Chrome to Firefox and IE (scary). Still no pictures on the dashboard.

I hope somebody can help me with this problems.
Regards

Timo