Unable to load world Error

Hi,
A several students have been working on Alice projects and when they tried to open their projects today, they received an "Unable to load world" error message. I've looked through the forums and haven't found any resolution.

It's worth mentioning that the classroom teacher indicated that the students who were having trouble were going back-and-forth between Alice and Alice Storytelling. He thinks they may have tried to combine some elements (backgrounds, characters, etc.), however, the students say the combined elements are not in the project.

Please advise.