Well, the saga continues with User Profile Service Applications. When you have more than one BDC service application the User Profile Service application will barf on it. You get Logging unknown/unexpected client side exception: SecurityAccessDeniedException. This will cause this application server to be removed from the load balancer queue.
The real error is an Access Denied in the WCF layer when it is trying to resolve across the multiple BDC service applications. I only want resolution of the ones I have access to, but evidentally it doesn't care and if any of them barf, the whole thing goes down. Bummer…
I deleted the other BDC service application hoping it would work, but then I get "There is not default Business Data Connectivity Service Proxy available for the given SPServiceContext", oh so yummy. So, I opened up the "Configure service application conenctions" and simply click OK.
One more try, and all is good…hmm…still some things to work out yet 🙁