Saltstack 2018.x fails to start after OS upgrade

If your Salt Minion fails to start with [salt.utils.process:754 ][ERROR ][5542] An un-handled exception from the multiprocessing process > ‘SignalHandlingMultiprocessingProcess-1:59’ was caught: Traceback (most recent call last): File “/usr/local/lib/python3.5/dist-packages/salt/utils/process.py”, line 747, in _run return self._original_run() […] File “/usr/lib/python3.5/asyncio/base_events.py”, line 411, in run_forever ‘Cannot run the event loop while another loop is running’) RuntimeError: Cannot run the event loop while another loop is running

You might be running Tornado version 5+

How to fix it?

pip3 uninstall tornado
pip3 install tornado==4.5.3