After some more tests with Cubase and a demo version of reaper, I think I have a hunch why one DAW hangs and one not.
Cubase, when you close a project, unloads all DLLs from the plugins used in the project (my guess is to save memory, as the core of current Cubase was developed 20 years ago when memory was expensive). This is what you see in the thread dump in my post above. Thus, the D2DFactory-Destructor is called (D2DFactory::~D2DFactory ), and this is where things go wrong, reference counters not being decremented, resources or locks not being released and the program goes into a deadlock (not a C+±expert, just a guess).
Reaper, on the opposite, seems keeps once loaded DLLs happily in memory after closing a project, ready for use in the next project. This makes sense, as Reaper was developed some years after Cubase and computer memory was getting cheaper. Average RAM size in 2000 was perhaps 512MB to 1G in 2000, but probably four times as big five or six years later. Memory saving wasn’t that big an issue anymore.
As I don’t have other DAWs around, I cannot test them, but my guess is that they work similar to Reaper.
Cubase just shows its age here (well, not only here tbh)