A quick thing to check would be if turning off TCP_FORWARDING_HOST allows the pool to work. Having it on is a supported configuration, but it'd help narrow down the debugging.
It'd also be helpful to know how you're specifying the new 8.3.6 collector on the old 8.3.5 client. (What was COLLECTOR_HOST for 118, what is the COLLECTOR_HOST for 121?)
- ToddM