Unable to Connect to PVS After PVS Server Upgrade or Downgrade

After an upgrade of PVS from 7.13 to 7.15, or a downgrade of the PVS server from 7.13 to 7.6, the ELM was no longer able to connect to it. Testing credentials would fail. Note, this probably happens in lots of other circumstances, but these are the two circumstances where we've seen this. If you...