I’ve submitted support ticket explaining in detail what is happening (Ticket Number: EU99650582), but in summary:
I own base game + dlc on steam and when trying to whisper someone in game it says “Unable to deliver the whisper to [name].” but the same thing occures when trying to whisper in desktop battle.net app (“There was an error sending your message to [user]. (7000)”. I have messages set to Everyone and i am also online in battle.net. For the moment the only way to message someone is to add them to friends list which is not convenient.