How to resolve "Derivative is infinite or NaN" error in MMC simulation with equivalent submodule circuit?

I am building an MMC simulation from scratch using equivalent circuits based on the simplified arm circuit shown in Figure below. My implementation uses a submodule equivalent circuit with multiple instances (4 submodules) as shown in my screenshot.

However, I’m encountering a “Derivative is infinite or NaN” error during simulation. The error appears to be related to my submodule circuit implementation, particularly around the time 1.0.0 seconds.