Hi Roel, the original FancyBox script was never built with long multi-line titles in mind. Probably because it’s not possible to accurately calculate the height of the title to adjust/scale the whole thing to fit into the browser viewport before rendering the lightbox.
My suggestion would be to disable the option “Center while scrolling” in the Window section. This will allow visitors the scroll down and read the complete title below each image.
The only other way is choosing “Title Position: Overlay” but I guess you already tried that and prefer Outside position…