I did a little investigation and I think I found out why:
The problem seems to be in the private method
It does the following to the tracker:
Now two threads do this concurrently this might fail:
T2: tracker.waitforService() -> returns null
The question is, what do we do about it.
I think this is an actual problem (not just in this test).
I think making the function synchronized is a bad solution.
I thought we could just skip closing the tracker (tried it, and did not encounter the bug since).