FYI I think I may have reproduced this by mistake, switching platform to Lumin. It didn't go away until I purged and rebuilt. I was hanging here:

[Inline Frame] mono-2.0-bdwgc.dll!mono_os_cond_wait(_RTL_CONDITION_VARIABLE *) Line 331 C
mono-2.0-bdwgc.dll!mono_coop_cond_wait(_MonoCoopCond * cond, _MonoCoopMutex * mutex) Line 90 C
mono-2.0-bdwgc.dll!mono_threadpool_io_remove_socket(int fd) Line 655 C
mono-2.0-bdwgc.dll!ves_icall_System_Net_Sockets_Socket_Close_internal(unsigned __int64 sock, int * werror, _MonoError * error) Line 788 C
[External Code]
mono-2.0-bdwgc.dll!mono_jit_runtime_invoke(_MonoMethod * method, void * obj, void * * params, _MonoObject * * exc, _MonoError * error) Line 2809 C
mono-2.0-bdwgc.dll!do_runtime_invoke(_MonoMethod * method, void * obj, void * * params, _MonoObject * * exc, _MonoError * error) Line 2915 C
mono-2.0-bdwgc.dll!mono_domain_try_unload(_MonoDomain * domain, _MonoObject * * exc) Line 2712 C
mono-2.0-bdwgc.dll!mono_domain_unload(_MonoDomain * domain) Line 2640 C
Unity.exe!UnloadDomain() Line 1761 C++
Unity.exe!MonoManager::CreateAndSetChildDomain() Line 1691 C++
Unity.exe!MonoManager::BeginReloadAssembly(DomainReloadingData & savedData) Line 1255 C++
Unity.exe!MonoManager::ReloadAssembly(bool scriptsCouldBeChanged) Line 1076 C++
Unity.exe!ReloadAllUsedAssemblies(bool scriptsCouldBeChanged) Line 464 C++
Unity.exe!Application::TickTimer() Line 1923 C++
Unity.exe!MainMessageLoop() Line 409 C++
Unity.exe!WinMain(HINSTANCE__ * hInst, HINSTANCE__ * hPrev, char * szCmdLine, int nCmdShow) Line 1072 C++