This issue has finally been resolved. 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.