can't set port forwarding, "The specified port(s) are being used by other configurations"

AC1200 WiFi Router (R6230)
Router Firmware Version    V1.1.0.110_1.0.1

I am trying to enable port forwardingfor multiple devices. I was able to create the appropriate entry for one device by using the "Add custom service" in port forwarding under advanced settings.

When I try to add another device I get the error message:

The specified port(s) are being used by other configurations. Please check your configurations of USB Readyshare, Remote Management, Port forwarding, Port Triggering, UPnP Port Mapping table, RIP, and Internet connection type

UPnP is disabled and its table is empty. Readyshare has never been used and is disabled.

Have already tried rebooting, factory reset, applying slightly older firmware (but the router forces you to upgrade before working).