My Mihome app is working again :)
The only thing I did, was to disconnect the hub/gateway from the power outlet over night.
The next day I re-connected it and after a while all my devices came online again.
Maybe I was just lucky with the timing(?)
Samme issue occured some days back (1. time) and still not working..
Since there seems not to be any way of fixing this (by myself), so I guess I just have to wait and see if the devices and MI gateway starts working again (?)