Start an ac simulation at f=0Hz
I guess I've found the answer minutes after posting the question.
Although I don't know yet why LTSpice does not allow a 0Hz .ac simulation, I know how to extract the DC impedance of my circuit (or as others may call it: the resistance). I basically run my .ac simulation from 1Hz to my maximum frequency and at LF (low frequencies) get a constant value which is basically the equivalent resistance of my system :)

Edit:
Also, after the clarifications of @ Bimpelrekkie, the .ac simulation does not run at 0Hz because &quot;it is a simulation of a linearized model of the circuit. The model of the circuit can change with the DC ( = 0 Hz) voltages and currents of the circuit, for example, the behavior of a transistor based amplifier can change depending on the DC biasing current. The linearized model is made for that DC current. Simulating at 0 Hz does not make sense then as that would change the model.&quot;
Hope I was clear enough for other users.