Had WPSCache nearly in basic settings and not caching when logged in. Lighbox Pop was set to automatic, centered position, and the size i think should not matter. Delay was 2 secs and it should pop up everytime with a Youtube-video and a contact form 7 inside. Think it has something to do with the delay.
Have it now in manual Placement and skip caching of pages with Lightbox.
But before i did this step i realised a strange behaviour after writing my post: The empty Lightbox showed only if i was logged in as Admin/ User and visited the page where an Lightbox should pop up. At this stage everything worked fine. When i then logged out and visited the Page from an other PC Lightbox was empty. If i did not visit the Page as logged in user before i visit it as anonymous everything worked fine.