You have to start the control panel of My Cloud.
Through the tab “Settings” in the “Utilities” select “Reset Only System” (or something similar - I have this text in russian).
The system returns to the factory settings. You information on the MyCloud disk will not be affected.
After this I changed the “admin” username and successfully registered at wd2go serever.
Many thanks for this tip, autodoc58.
I have tried many things, including adding the e-mail domain in CAPITAL letters, but without success.
Your tip regarding the ports was what worked for me.
It is a pity that none of this is in the users manual or other official communication from WD.