Students unable to view documents associated with imported quiz

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I imported a quiz (QTI file) exported from another Canvas course. Each question on the quiz is supposed to "file preview" a document (a PDF) on which the question is based. When I open the quiz in preview mode, it works just fine; the documents appear in the question box.

But when I open the quiz in "student view" (or when a student opens the quiz), instead of a file preview, I see "The document preview is currently being processed. Please try again later." If I click the download button for the document, Canvas says, "The folder '[folder name]' which includes this file, has been locked."

I went into Files>Quiz Files to examine the documents. All of the files associated with the quiz are set to "Only available to students with link." Clicking Publish>Update does nothing; the files remain "Only available to students with link."