I have encountered this issue twice so far this semester. Once on a homework set, and today on a gateway assignment.
Both times, the problem set was assigned, and a number of students had already completed work on the set. Then (for unknown reasons), the path to the source file for a bunch of the problems just disappeared.
Today, for example, I had a student taking a gateway quiz with no trouble. About one hour later (still well before the due date), I started to receive emails from students indicating that there was an error (source file unavailable). When I opened the set in the HW Sets Editor, there were still 14 problems listed in the set, but only the first 7 had paths to the source file. The rest were all blank.
Any ideas about why this might happen?