Yes, there are two others in the past who have reported that error.
https://community.wd.com/t/problems-after-fw-upgrade-wdmycloud/93232/12
https://community.wd.com/t/share-function-failed-400099-media-server-error-400162/95322
Some basic suggestions:
Obviously try restarting the My Cloud to see if the error clears.
If any media has been added to the My Cloud recently remove the media, its possible Twonky is choking on a media file.
If the My Cloud firmware has been modified in any way, like to fix sleep issues or add modules, remove those modifications and see if the problems clears.
Try a 4 second reset and if that doesn’t work a 40 second system restore.
