Make sure you are connecting to the My Cloud using SMB and not AFP. Apple and WD have depreciated AFP support. WD indicates; Apple File Protocol (AFP) is no longer supported on My Cloud OS 5 firmware 5.19.197 and higher. WD Support has the following Knowledge Base Article which has additional suggestions:
Also note there is a specific subforum for the OS5 Pro Series of devices where users more familiar with that device may be able to help with additional issues with the PR2100.
Can you post a screen shot of the error message you are seeing or post the exact full wording for it?
Does Finder -> Go -> Connect to server -> smb://<AlternateUsername>@wdmycloud work to access using the alternate user name and password (when prompted to enter a password)?