Thank you for your answer, Philipp.
As I understood, I definitely need to assign a serialVersionUID for every message type I am adding to the source code. My question is now: How?
What do I have to keep in mind when assigning it? Do I have to avoid declaring one that is already existing in some code of the simulator? How did the developers of the simulator come up with such huge long (datatype) numbers? Can I just use a random long number with the same length as one existing serialVersionUID?