I would like to know if there exists an appropriate UML-diagram to depict sth I would describe as "object-flow", similiar to the example below:
It's very similiar to an activity diagram, only the focus is on objects/entities here, not on activities. IMO, an activity diagram is not the best choice to model the above nicely, and I don't know what else to chose. I would like to use UML however, whenever possible.
Do you have any suggestions?
Looks pretty much exactly like a dataflow diagram. But they're not UML. I think Activity Diagrams are the closest you'll find in UML. Activity Diagrams do support Object Flow specifically - have a look in the spec. But it looks like a DFD is better fit given your example.
hth.