Attempting to connect results in SystemError('_PyEval_EvalFrameDefault returned NULL without setting an exception')

I have two separate projects that I want to communicate using MQTT. I can start up one of them and have it connect a Paho MQTT client without errors, and even communicate with it on the command line. After that, when I try to call client.connect in the other repo, it results in the error shown above. There are sometimes when it doesn't fail on that, but then fails with the same message when attempting to subscribe to a topic. The version of paho I'm using is 1.6.1.

The specific line triggering the exception is line 649 of client.py, at the top of the function _sock_send:
return self._sock.send(buf)
If I put a debug breakpoint there can run "self._sock.send(buf)" in a console, it can return a value of 14 without problems, but then immediately afterward when I try to step into or over that line, the exception occurs.