UDMP WAN2-only Connection Problems

I have a UDMP that I cut over to using the WAN2 port since I have a fiber net connection. This appears to be working, but I end up with ongoing log messages of the form.

“Multipath routes not supported, got 218173 nexthops for route ::/0 via fe80::dead:beef:70:1 dev eth9”

Does anyone have a documented known-to-work procedure for using WAN2 as the sole WAN connection? In its current configuration, port 9 (WAN) is enabled but reporting disconnected, and the WAN2 port configuration in the controller settings appears as “failover only”.

Im getting between 2 and 4 of the log messages per second, which can’t be a good thing. The usual googling and forum checking has not yielded anything useful, just different variations of using the WAN ports in a failover configuration.

ubnt support has been about as useful as expected, and fairly obviously have not even looked at the logs and config I sent, based on the questions they asked (eg what version of firmware)

Any thoughts?