[ERR 21:44:19.803] MechJeb module MechJebModuleAttitudeController threw an exception in OnFixedUpdate: System.ArithmeticException: NAN
at System.Math.Sign (Double value) [0x00000] in :0
at MuMech.MathExtensions.Sign (Vector3d vector) [0x00000] in :0
at MuMech.MechJebModuleAttitudeController.OnFixedUpdate () [0x00000] in :0
at MuMech.MechJebCore.FixedUpdate () [0x00000] in :0

vesselState.angularMomentum.Sign() is getting NaN back.

It is in OnFixedUpdate() so it isn't in Drive() so vesselState has been updated.

That suggests something in UpdateMoIAndAngularMom is getting a NaN value, probably back from parts.

I would hazard a guess this is an interaction with some other part mod and isn't a bug in MJ (the stock vessel.MOI might protect against this and/or still be bugged?)

Should probably go back to the stock vessel.MOI in 1.4.x since I think the MOI orientation bug got fixed there.