Arduino getting a bit of power after disconnecting supply
Ideally, the switch in such an extension cable switches the live and the neutral lines.  Often, only one will be switched because single pole switches are cheaper.

If only one line is switched, you want it to be the live wire.  But, live and neutral are pretty much undefined in a german outlet.  You can never be sure (without measuring the voltage to the ground wire) which pin in the outlet is live.

So, you switch the extension cable off, and maybe it breaks the neutral instead of the live wire.  You then still have 230VAC going to the outlets, but no current flows because the neutral is open.

Except that your outlet strip also has that 5V power supply in it, and perhaps other leakage paths.  That allows a small current to flow through the neutral wire from the outlet strip back to either the real neutral or to the ground wire.

This gives your powersupply a little bit of current to play with.  Not enough to operate properly, but enough to output just enough voltage and current for the Arduino to start up - and it draws enough current to short out that little bit of leaked current, so it shuts off again.  Then the whole thing starts all over again.



Unplug your extension cable from the wall, rotate the plug 180 degrees (to swap the pins in the outlet) then plug it back in again.  Now see if your Arduino still blinks.