Hi Ana,
Glad that the new SynergyController feature is working for you! And thank you for sharing your example.
The error you are seeing is related to Simbody's Assembler, which will tries to update the model coordinates in order to satisfy any kinematic constraints in the model. This error sometimes pops up when the defualt model coordinates violate the kinematic constraints (i.e., during an initSystem() call).
Moco uses its own method for enforcing kinematic constraints, so if the Assembler isn't failing and interrupting your simulation, then you can usually safely ignore the error message.
Best,
Nick