Do the instructions indicate that you should create an ibay named 'nextcloud'?
There is nothing in instructions about creating an iBay
I'd try removing that ibay to see what happens.
Can't remove iBay from domain, there are options to assign different iBay to the domain, but not remove it.
I did assign Primary iBay as a test and I see generic primary iBay notice: "This web site is under construction"
And same as before, I am able to connect to NextCloud from internal network, but I see: "This web site is under construction" when connecting from outside network. (entering same domain in both instances:
https://mydomain/nextcloud )