@Miguel Correa: The

comp.out

output has entries like this:

Net: dummy_11                              |Net: sky130_fd_pr__nfet_01v8_4MHCRP:XM3/a_
  sky130_fd_pr__nfet_01v8/proxy4 = 1       |  sky130_fd_pr__nfet_01v8/(1|3) = 2

Since you specified the schematic netlist first (usually it's the other way around, that's why it's called "LVS" and not "SVL"), the left-hand side is the schematic netlist. The "proxy4" pin name indicates that there was no pin 4 (bulk terminal) on the nFET ("M3"). If I go back to the netlist source, I see entries like this:

XM3 VSS out VSS sky130_fd_pr__nfet_01v8 L=10 W=100 nf=4 m=1

This is an invalid entry. There are only three pins (VSS, out, VSS) passed to the nFET, which is a 4-terminal device. Normally xschem won't do that, but my guess (without being able to see the schematic) is that you used one of the 3-terminal device symbols, and the bulk terminal, which is a property of the symbol rather than a pin, was left as an empty string, instead of being set to "VSS".

You're right! This is my schematic:

You got an error, not the same error. If I correct the bulk terminals in the netlist and re-run, now I get output like

Net: sky130_fd_pr__nfet_01v8_4MHCRP:XM3/a_2029_n250 |(no matching net)
  sky130_fd_pr__nfet_01v8/(1|3) = 2                 |

This indicates that the net is connected to two device sources/drains. Since the devices are fingered, the contact between any two fingers of the device is already connected to two device sources/drains. That means that the fingers of the device aren't connected to anything else, so you're probably missing contacts between the device fingers in M3, M4, and M5 in the layout.

Thank you very much for your help! Everything works fine now 🙂