It looks like the error message is a WeBWorK bug, probably due to some recent changes to the way that set names and problem numbers are verified. I will report the error in the WeBWorK issue tracker.
I suspect that the intention of the homework was to use the "Update" tab rather than the "NewVersion" tab to edit the problem in place (each of you has a separate copy, so there is no need to make a new copy). Of course, I haven't read the instructions, so I might be wrong. :-)
PREP 2015 Question Authoring - Archived
NewVersion
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