I think I found the bug but don’t know how to fix it yet. Due to my research it is 100% caused by the original hueman theme. The layout container adopts the height of the “highest” image in the gallery. That’s my guess, that’s the bug
Hey, Plugins weren’t a problem. It was caused by my Hueman-Theme. I switched to a WP default Theme, uploaded everything, switched back to Hueman and now it seems to work!