Why is it not possible to measure voltage between one battery pole and mass/ground
This illustration may help:



Figure 1. Without a ground reference the car screw-jack is unable to provide any lift.

In a similar manner to the car-jack analogy, a battery without a ground reference cannot provide any lift / thrust / force / current. We need to close the circuit.


  So you have one DC multimeter and attach one pin to lets say the plus pole of a battery and the other pin to ground/mass (&lt;- is there a difference between those btw.? haven't found anything googeling). 








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. In (a) the battery is not referenced to ground other than through the meter. No current will flow and the voltmeter will read zero. (b) In this case the circuit is completed through the ground and current can flow around the loop through the meter.

In electrical circuits ground is some reference point and, by definition is the zero volts reference. This is analogous to a surveyor picking a zero reference from which to reference all his/her other measurements. Note that voltages can be positive or negative with respect to the reference point.


  In my understanding there is a positive potential on the positive battery pole. 


It is only positive with respect to the other pole. Again, a height can only be positive with respect to a reference height.


  Shouldn't the multimeter show the difference in potential between those two points of contact?


The fact that there is no circuit, the act of connecting the multimeter will connect the battery + to ground. In practice you will see a stray voltage reading due to capacitance between the battery and the earth but there is no power behind it and it couldn't, for example, light an LED.

Have a look at my answer to 
Few questions about basic concepts in electronics for more on this.