Upgrade of mycloud os3 to 5 did not work

Folks, what is that?!
I started the upgrade from mycloud os3 to 5 via dashboard of my ex2ultra. Got the confirmation, that the upgrade was successfull via email. Firmware version is 2.40.157 /
And now? Nothing. Device can not be found via nor http://mycloudex2ultra/ and it is not recognised anymore by my router. A 40sec reset failed, ex2ultra is still not reachable.
And now?

@Markus.Huebner Are the Power and Drive LED lights red or blue?
Are the lights on the back of the unit where the Ethernet cable plugged in illuminated or not?
Can see the My Cloud in Windows Samba or macOS finder?
Have you retried rebooting the network router?