Without the SD card the Raspberry Pi's green led shows the pattern you described. It does not boot after following your instructions.

I only have keyboard and mouse connected, doesn't make a difference if I have them plugged in or not.

Also this behaviour doesn't show up on reboot, only shutdown.

It's the same thing if I unplug at the mains or disconnect the power cable from the Raspberry Pi.

Once I shutdown safely and switch the power off at the mains socket, the red LED turns off. If I then turn the mains back on, the Raspberry Pi won't boot. If I wait a while before turning the mains back on, then it will boot. I know that if I wait 1 hours it will boot, I know that if I wait 5 minute...

The official one from Raspberry Pi.

Hi, I have a Raspberry Pi that boots and works fine, but if I shut it down via any method it won't boot again until I wait some time. I usually wait one hour and it works. I don't touch any cables or do any re-plugging of any component, only the power connection at the socket. When I try to boot it ...