Menu Settings loading Problem at creating new Form

When I try to create a new form, everything works fine. The last menu item Settings also shows everything.
However, as soon as I fill a pull-down menu with content from the database (e.g. user names) and click on Settings to configure how the content of the form is to be written to the database, LOADING stops and nothing happens.
If I delete the pull-down menu with the database content, Settings loads completely normally again.
The content in the pull-down menu is displayed completely ok under Preview. There is no problem.