"File drawing-1 is related to a drawing and could not be found" error

Hi,

I’ve taught animation via Toon Boom Studio for a few years, and encountered a recurring problem. Students often get the "“the file drawing-1 is related to a drawing and could not be found” error message when opening their projects, and I don’t think they’ve moved, renamed or deleted the files the software’s searching for. Does anyone know of another cause/solution for this problem?

Thanks!
Tom