Thanks for suggestions, I will try and make an empty project with just two empty layouts, one with the images and (hopefully) see what causes it.
I read that using the tilebackground for bcgs with large images is less effective than sprites, because it is optimized to repeat the image, but I will give it a shot.
I have the large background image split into small sprites that I arranged one to another, which I found in the documentation to be the suggested way of dealing with it.
I can post the code, but it is really simple, just as I described - but I don't want to post the images, it is a commissioned work of an artist.
In any case, is there a way of making a loading layout that would load the graphics after the application is run? So that you would click in the menu on "game" or something and the graphics would start loading only then.