as it turned out a modification of the theme css code was responsible for the broken OSM render - for testing you could just change the theme of your (really nice!) site and check, if the maps are still broken then.
'Fullscreen Galeria' itself is working properly as I assume, I think you have to check/change/take back some mod in your css code of the theme then to bring the proper map render back to life, which might not be easy... :-/
Hope this helps a bit - good luck!
Best regards from Cologne, Germany