This issue has finally been resolved. After much prodding on our end for Canvas technical support to actually investigate what might be causing Canvas to generate the "duplicate nonce detected" error, Canvas was able to identify that our WeBWorK server was posting times that were more than 1 minute ahead of the timestamp Canvas had. Apparently Canvas only allows for 1 minute ahead and 5 minutes behind. After syncing the time on the WeBWorK server (as of today it was about 90 seconds ahead of the "official" time), I was able to grade a problem in WeBWorK and have the score successfully sent to the LMS. I guess on May 16 between 10:43 pm and 10:46 pm when grade feedback stopped working, our WW server's time must have crossed over the 1 minute ahead threshold.
WeBWorK Main Forum
error during LTI reporting of grade back to Canvas
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