SDL_Flip deadlock - c++

Question

In my application, I've got a simple Execute method that runs the application,
greatly simplified, it looks something like this:
App::Execute()
{
Initialize();
SDL_Event event;
While(Running)
{
while(SDL_PollEvent(&event) > 0)
{
HandleEvent(&event);
}
Render();
}
}
and Render() looks like this, again simplified:
App::Render()
{
SDL_FillRect(MainSurface, NULL, 0x000000); // MainSurface is declared inside the class
// mCurrentScreen->Render(MainSurface);
SDL_Flip(MainSurface);
}
Now this works fine when the debugger is attached, but when I just run the executable,
the application freezes, attaching gdb to the process, I can tell the main thread is stuck inside SDL_Flip().
I have managed to narrow this issue down to me launching a separate thread that listens for clients using sockets, as commenting out the start of this second thread, makes it work in all modes!
The thread is started like this inside the class 'Server':
Server::Start()
{
mThread = std::thread([this](){ListenThread(this); });
}
ListenThread does the usual socket setup
getaddrinfo(), socket(), bind(), listen() and finally, the call to the blocking function
accept().
Again, this server, and the rendering and everything, works perfectly when a debugger is attached, it is only when launched "directly" that it hangs (forcing me to KILL the process to stop it).
The Server and App has no common data structures, Server doesn't know anything about App,
the only relation between them two is that App sets a callback function on Server.
mServer->Callback = [this](Command cmd){ this->CommandReceived(); });
Edit(Append): This only occurs on my PC, I have the exact same setup (ubuntu, gcc, sdl version etc the same) on my laptop, and it works fine on my laptop.
Note sure if/how it's relevant, but I'm running Ubuntu inside a Hyper-V virtual machine on Windows 8, if that's any help.