I've been moving into an apartment for three months now and have two circuits here, if I connect a device with a neutral conductor to one circuit, such as a deep fryer, laptop charging cable, washing machine, etc. In the second circuit, all devices are working properly.
The sockets (faulty circuit) are 2x in the living room, 1x terrace, 2x in the basement.
How can I check which socket is hiding the error, which is why the FI keeps flying out?! And which error is likely to be present here and can it be easily fixed?
Sounds like a connection between N and PE. Somewhere in the circle there's a connection between N and PE. If no device causes this error or the error only occurs when several devices are switched on, then it is a corresponding contact resistance. Part of the current does not flow through the neutral conductor but rather through the PE, which is recognized as a fault current.
Only an electrotechnical specialist can search for errors here. As a loan, you should by no means go looking for errors here!