Yes. It was changed I believe at the introduction of Rack v2, so that the Audio module drives everything in Rack in a kind of “pull” fashion, which is different from v1 and earlier. So yes, even if you think you don’t need it always have an audio module in a patch, even if it’s unconnected. It doesn’t really use any CPU.
What surprises me in this case is that anything worked at all without an audio module. In thinking about this case I suspect what might happen is this: The audio module actually drives the audio interface, with respect to samplerate but also with respect to clock and sync of the interface. Since the MIDI out being used is sitting in the MOTU it might make sense that the audio module should be connected up to the MOTU, so that everything (MIDI out from Rack) is synced up and clocked uniformly. However, it really baffles me to hear that what worked is connecting the audio module to another interface than the MOTU.
@timnordberg - what happens if you connect the audio module to the MOTU instead?
Hang on @timnordberg , why did you suddenly drag Reaper into this? You didn’t mention that originally. Do you have Reaper open at the same time, connected to your MOTU? Because then it is controlling and clocking/syncing the MOTU where your MIDI is sitting. Can you see the problem? At worst it might have been fighting with Rack for control of the MOTU (and hence MIDI) and that would make things wonky, and then it would make sense that changing to another interface in Rack would solve the problem.
I’m a bit confused about all the pieces at play in your setup here, and maybe you’re not telling the whole story… anyway, if you’ve got it working that’s what matters.