Hi there, I had a fight with getting RDP working in Remmina, and while I did end up resorting to a third-party app in the end. One thing that was screwing with me was that I did not notice that the PW I set on the local pc when setting up the RDP connection was always resetting itself to a different & random PW after each restart of the pc. Meaning that the PW was never the correct one.
Just check to see that this is not the case for you.
I had other related issues at the same time as well - here is the forum thread RDP Z to Z fails - Z to Win10 good
and here for the keyring/PW issue - Editing Keyring and PW's for RDP access w permanent PW w/o monitor - Success
cheers, that's my 2 cents - Good Luck
CD