Your modulator has a condition where there is a short circuit across the DC source. You can place a small resistor in series with the lower DC source to suppress the error message and then observe the source current spiking at 0.015 seconds (and periodically after that).
The issue relates to the with the “>” vs the “>=” in your modulator for the neutral-connected switches. That’s why it occurs only when your modulation index is exactly 1 - the carrier and the reference waveform are both exactly equal and your neutral-connected switches briefly toggle. I didn’t review the full modulation scheme, which you should do, but for the particular error message the highlighted component is the culprit (set to “>” from “>=”.
