What does the "Add DC Path to Ground" in the Preferences dialog box do?
In order to solve for the DC operating point, it is necessary to have a finite impedance from every node to ground. The necessity for this can easily be seen because a circuit without an impedance to ground has an infinite number of solutions and this poses something of a problem for the simulator. This option avoids the problem by automatically adding a 1/Gmin resistor to any node that is missing the required path to ground. Usually it is best to disable this option and provide the impedance manually so that you know what node is involved and can better judge how to provide it with a path to ground.