Ok, please try two things:
Check the interface name on Signal K for the CAN interface, if it is can0 or vecan0, please try vecan1. Make sure to restart it after you make a change. See if you can connect it to first port (even temporarily) and if it makes any difference.Independently, the first port is galvanically isolated, the second one is not. For NMEA connection, Victron's strong recommendation is to use the first port.