homepage url will change itself

I tried to create an One-Pager. Therefore I had the original homepage as an “under construction site” and created a second page where I put all the content that shoud be the real homepage later. Now I wanted to deactivate the “under construction site” and put the real homepage active, but it will instandly change back to the “under construction site”. I don’t understand why, I changed it in the customizer, but it will turn itself back. When I change the names of the Sites it doesn’t matter and even the permalinks won’t effect the outcome.
“Startseite” shoud be the right page, but it always changes back to “Im Aufbau”.
Please help!