“Admin Approval” resetted after saving secutiry settings

Dear support,

we setted up the pie plugin in 1:1 copy of the LIVE-website.
It worked well.

Redoing it on the LIVE-website, an very strange error happens:

Whenever i change Settings >> Secutriy >> New user verification from “Disabled” to “Admin Approval” or “Verfiy Email Address”, the setting is reset to “Disabled” each time i save the settings. There is no message or hint why this happens.

It works like expected in the TEST-environment.

I cannot disable plugins / themes one by one to test the stuff from others.

So, what is you suggestion?
How can i enforce enabling the setting?

Thank you,
Thomas