I am with a problem and I don't know how to solve. The following simulation just works with the capacitor, but it is connected to nothing.

If the Va current goes to zero (and this might happen when V(A)=0) you have a division by zero. The error without the capacitor and no error with the capacitor is just coincidence. SPICE simulators always have some small numerical errors, so the simulated current through Va could be exactly zero or not