I have been reading several posts with similar problem, didn’t find the solution. Finally I solved the problem going to WD my cloud User Interface. I disabled DLNA (wait for the configuration to save the changes) and re-enable it. This seems to reset the DLNA server and sort out the problem.
Disabling and re-enabling stops the Twonky service and restarts it, which rebuilds the Sharing settings in the Twonky configuration file. This is a third level of severity of restarts, but one I forgot to include in my FAQ. I’ll change it, thanks.