It appears from quick googling that Windows Server 2012 has some problems backing up to non NTFS formatted NAS drives like the WD My Cloud. With respect to the WD My Cloud you may want to review the following thread where a similar problem of being unable to backup to the WD My Cloud from Windows Server 2012 was discussed and one possible solution was linked to in that thread.
WD My Cloud 4GB And Windows 2012 Server Backup
The other option is to possibly use a third party backup software program and not the built in Windows Server backup program.
