I have run into this problem as well. In no case was I able to control multiple boilers on a water loop serving heat pumps. Similar error as you are reporting. Tested the multi-boiler setup (with equipment controls and load management) on a hot water loop serving heating coils and it worked fine. My workaround ended up being a spreadsheet calculation to determine the composite efficiency of the multiple boilers, based on running them on a hot water loop in a different model, and applying that to the single water-loop boiler. Not very satisfying but I had to move on.

Evan Riley PE, BEMP, LEED AP BD&C
Sr. Energy Engineer, Building Sciences Studio | Mason & Hanger
O 804.822.3008