Starting the multiplex coordinator in single-node mode (-iqmpx_sn) and read-only (-iqro) performs some initial database checks. For coordinators, the server must reset an identity cookie before you can use the multiplex.
Once you successfully restart the coordinator with iqro 1 and iqmpx_sn 1, shut it down and restart it without any special switches.
start_iq -n mpx_main -x tcpip{port=62631} -iqmpx_ov 1 d:\work\iq-127\mpx\main\main.db