Problem with Large domain

Hi

I am simulating propagation through a simple waveguide in 3D.
The simulation is working perfectly as long as the length of the waveguide is below 10um. When I increase the device length to 10um, the simulation continues and reaches the convergence, however at the final step an error message appears. as below.

Failed to find a solution.
Very ill-conditioned preconditioner.
The relative residual is more than 1000 times larger than the relative tolerance.
Returned solution is not converged.

I have attached the file. I would appreciate any suggestions.

Thank you

Kapil