This could be a number of things. Basically, the solver is telling you that it cannot successfully solve the system of equations. In your case, you might have the velocity source trying to specify a non-zero velocity on the ground. By definition, the mechanical translational reference has v=0. If the velocity source is directly trying to specify v=X where X is non-zero, on the ground, the equation will fail. Mathematically: v = 0 v = X X ~= 0 There is no solution to these conflicting equations.
I am speculating that you may have intended to have some flexibility between the ground and the source, possibly with a translational spring.
