Resolved! Tried --> GlideDialogWindow: Advanced Popups Using UI Pages --> <Page Not Found> / Loading...
I tried to add a Pop-Up according to the following tutorial (Kingston Release)https://www.servicenowguru.com/system-ui/glidedialogwindow-advanced-popups-ui-pages/ The Dialog appears but the error message is: Page not found...or it stays at Loading......
