Two active bugzilla reports involve a problem's link to a "units" file producing an error message.
http://bugs.webwork.maa.org/show_bug.cgi?id=2704
http://bugs.webwork.maa.org/show_bug.cgi?id=2828
After several failed attempts to work-around that ongoing bug, I discovered that copying
(path to OPL)/Michigan/units.html
into
.../webwork2/htdocs
(rather than down one level into .../webwork2/htdocs/helpFiles) would let the problem's javascript do its thing.
FWiW: comments in bugzilla suggest a more enduring fix (and I will add a comment there about this post).
WeBWorK Main Forum
temporary fix for missing "units.html" file
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