Adding new Port Forwarding rules always returns "Input Errors"

No matter what options I choose when attempting to define a new port forwarding rule, I always get the response "Input Errors".

This clearly worked in the past because I have other rules configured.  I started bumping into this in the last couple of days as I want to make some tweaks / adjustments to what forwards there.

I can still disable and delete existing rules, but I can't create new ones.

Looks like the router is returning a 400 with no content.

My router details:

Anyone else seeing this?  Any help would be greatly appreciated.

Thanks!

Ryan