[SalesForce] Why a Community in Sandbox shows only “URL No Longer Exists”

I have a community in Production-Org up and running.

• It's a Lightning Community based on Napili-Template
• the Production Community URL is https://MYDOMAIN.force.com/Community/s/

Now I've spun a new Developer-Sandbox (no data) today.

• Name of my new Sandbox is "comm0922"
• The Sandbox has copied the Community to the new Sandbox
• The new Community is listet at All Communities on comm0922
• The new Community is editable with the Community Builder
• The new Community has it's own URL (different from Production-Org) https://0922comm-MYDOMAIN.cs89.force.com/Community/s/

But the Sandbox-Community-URL only shows this

Question

What are the exact steps I need to take on a blank new Developer Sandbox in order to see a working copy of my production community? Why I've getting the Aloha-style error, while this is a Lightning community?

What have I done so far?

• checked this Community URL No Longer Exists
• re-published Community on Sandbox (completed successfull)
• still "URL No Longer Exists"