My plugin runs once but not twice

I have a plugin that calls C++ code. When the game starts, it calls this extern C++ function:

 void startPlugin() {
   MyClass::instance = new MyClass();
   MyClass::instance->process();
 }

The process method runs an infinite loop that processes data and goes on, as long at the keepRunning member is true. Because it runs an infinite loop and blocks, this method is called within its own thread.

When the game ends I run this extern method from C#:

 void stopPlugin() {
   MyClass::instance->keepRunning = false;
   sleep(1); // Make sure the process loop is done. It should take less than one frame, but just to be sure...
   MyClass::cleanup();
 }

Which refers to this method:

 void MyClass::cleanup() {
   delete instance;
   instance = NULL;
 }

Based on what the console says, these methods are indeed run.

On Mac, this works the first time. I can start the game in the editor and stop it in the editor and everything works fine. But if I try to run it a second time without restarting the Unity editor, the whole editor freezes. Sometimes it takes three times instead of two, but invariably the second or third attempt to play freezes Unity completely. The cursor changes to the spinning pinwheel.

The log says:

 Starting.
 
 (Filename: /Applications/buildAgent/work/d63dfc6385190b60/artifacts/MacEditorGenerated/UnityEngineDebug.cpp Line: 49)

 Receiving unhandled NULL exception
 Launching bug reporter
 Obtained 37 stack frames.
 #0  0x00000090019390 in _platform_memmove$VARIANT$sse42

And then the stack trace brings up the function that calls the extern function.

Trying to join the thread that was running startPlugin freezes the editor on stop. I don't think it should do that. After all, at that stage, the loop is over and the instance cleaned. This is not guesswork, there are visible side effects: ending the loop turns off the computer's camera and I can see the light of the camera go off. Also a debug message is sent in C# after the call to startPlugin returns. So there is no doubt that the loop is over.

Am I doing something obviously wrong?