Thanks a lot for the hint!
As this was the first time I was dealing with threads in ROOT, I completely missed one crucial piece of information - that I need to explicitly turn on the multithreading support.
It works now with ROOT6 by calling ROOT::EnableThreadSafety() as well as with ROOT5 by calling TThread::Initialize() anywhere before creating the threads, eventhough I’m not using ROOT’s implementation of threads, but pthread directly.
So ROOT6 is not really needed, but the changelog you pointed me to contained all the necessary information 
Thanks again!
Tom