WordPress Membership Plugin
I was able to solve the issue myself. I explain below what happened for the help of others.
Rational is, that I got mainly confused by the page behavior as logging on as new established user I – of course – had no admin rights and hence the menu disappeared according to the setup of the plugin. Now, after logging of and logging in in the wp-admin side under my admin name the admin manes were right back where they had to be.
Regards, Klaus