@sgold a few updates from today:
I am almost able to reproduce the error within 5-10 minutes. I jump between 4 different airports very heavily and call System.gc() every ten seconds. This seems to provoke the Exception.
There is no guarantee however, sometimes it just works and sometimes it does not. (Probably I should try it only on Mondays.)
Additionally, I found another thread. The difference is, that I do not manually dispose stuff, but I think the same thing could be happening. I don’t want to jinx it, but the additional call to ref2.clear() seems to fix the problem on my side too. I’ll keep you informed in any case.
I would appreciate your thoughts however, because I still do not understand the problem 100%.