Error: Connection to the database server failed: Operation CREATE USER failed for

I'm experiencing this issue when creating a mysql database and user together, or when adding a user to a database:
Error: Connection to the database server failed: Operation CREATE USER failed for 'db-user-name'@'localhost'

Probably related, if I try;
plesk repair mysql
from ssh, I get errors creating absent users for existing databases:
The user db-user-name is absent from the database server ........ [ERROR]
Recreate the user db-user-name with default permissions and default...

Error: Connection to the database server failed: Operation CREATE USER failed for