### I get the following error : “[-477] qdd is Nan”. What is the problem ?

It means that one of the accelerations from your generalized coordinates is “Not a Number” (i.e. infinite) during the direct dynamics, after the time integration.

Often, this error comes from wrong values in the user files functions. Check the values of your user forces/torques, for example in JointForces/ExtForces/etc.