Ok, I found the answer. It works but it's a kind of circuitous way to get the global mean.
First of all you need the total extent of the image. Either possible by automatic extraction or manual selection.
Afterwards you need to apply the Zonal Statistics Tool and define the zone as your raster's extent.
As value raster use the raster file where you want to derive the global mean from and the statistics type has to be, in my case, MEAN.
The result is a new rasterfile with meanvalues only.
