You might have one of the Hots and Neutrals mixed up on one of the outlets.

CAREFULLY check the "eyes" of the outlet to the "mouth" (earth) of the outlet with your meter.

On the incoming cord, Black or Brown = Hot, Blue = Neutral, Green/yellow = Earth.

What Vid1900 said, the hots and neutrals are not the same in both outlets...be careful

He is in Denmark and has a totally different system than the US. Denmark uses the EU standard 230v 50Hz. Start by checking the ground continuity in your receptacles and cords. I don't know if there are European receptacle testers like we have here, but they can be very helpful.

Yep, we got that, thus the :

If it was USA it would be Black=hot, White=neutral, Green=GND

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Did someone cut off the grounds from your male plugs?

I'd still guess that your Neutral and Hot is reversed on one of the outlets.

The jack on the back of your machine is an IDC type like the one below, yes?

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Or I guess another way to ask is if your cable is the one below, but with the ground pin cut off?

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If I were you, I'd buy new cords for your machines that have the ground pin intact. The ground is there for protection.

Most computer stores have 1000s of these for a couple of bucks.

BUT, I still think that one of your two outlets is wired backwards and the Hot conductor is on the wrong side.

Carefully put one lead of your meter in one hole and the other lead in the ground hole and note the difference in your two outlets. If you are handy, pull the outlet and swap the hot and neutral on the incorrect one - if not, call an electrician - it's a 2 minute job.

I don't think so.
I have had the same problem (Netherlands, also in Europe) because out here a lot of older houses have just the hot and neutral on the socket outlet. Just some rooms e.g. kitchen have the earth as well.

I measured about 70V, but it has no power. You can feel it when touching one pinball which does have the earth connected and the other one doesn't. I always suspected the capacitors inside the linefilter.
I pulled extra earthing wires and replaced the socket outlets for ones with an earthing connection. That solved the problem.
Years ago at my rented apartment it was not allowed to modify the electricity, so I connected the legs with each other and connected it to the central heating. That also works (as long as the wire stays connected...).

If it were full power from the socket outlet the kiss could be deadly and when touching both machines with either hand could also be deadly. And connecting the both machines with a wire would create a short circuit.

Yeah, houses from the 1950s are like that in the USA also.

But even though one hole is slightly smaller on the socket, you can still mess up and have the hot accidentally on the larger blade.

I've also had customers get shocks touching two games from different outlets, when someone "grounds" one of the circuits to a nearby water pipe, rather than returning ground to the fusebox or circuit panel's neutral bus .

This is often in a commercial setting where 100s of modifications and remodels have been done in an older building.

I also had a recording studio service call where the owners came up with a crazy grounding scheme that was going to kill someone sooner or later. In the name of "good sound" they though that they needed to run and bunch of separate grounds rather that return to the Neutral Bus. At some locations in the lounge there was 65v between the Neutral and ground plugs.

My best friend is one of those $5 circuit tester plugs that tells if the ground is functioning, or the Hot and Neutral are reversed. I check on every install, first thing. You can also check with your meter, but the plug is faster.

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Are houses in the USA also equipped with residual current devices? We have them and they trip at 30mA earth leakage. The idea is that someone touches a hot wire, and if the current flowing through your body (to earth) is more than 30mA the power is automatically switched off.

@DEN: do you have a device like that in your home?

Yes, we have Ground Fault Circuit Interrupters anywhere within 5 feet of water (kitchen, bath, laundry, bar) for the last 20 years , and more recently code has required Arc Fault Interrupters for bedrooms (these open when any arcing is detected).