Thanks very much for the suggestions. In the end I tracked down the problem by successively deleting problems until I found the culprit.It then turned out that webwork was unable to find rank.pl, which is part of the open problem library -- and which was found when I previewed this question. Not sure what caused this error but as a work-around I found the code for rank.pl in the github repository and copied it into the question. Now both webwork and I are happy.
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