Hey Armin, its me again! Sorry :smile:
I updated on 1.9.0 today. I don't know if this only occurs on the site I'm working on or in general but since the update a small button referring to lightbox appears in the upper left corner while loading. After the page load is finished it disappears. I checked Custom CSS, I switched off the lightbox-addon via the menu and tried different browsers (also happens to appear on iOS). Nothing fixed it.
Here are some screenshots: