Posting here to help others hopefully spend less time figuring this out.
I had some issues where remote desktop stopped working. I could connect, but it would not accept my username password. Everything looked OK...the user had access, remote desktop was enabled, etc. It turns out that Windows 10 merged my local computer account with my Microsoft account. It resulted in Store errors ("Something went wrong on our end") and Remote Desktop not accepting my password. I tried using my local password, my Microsoft password, etc. Nothing worked. I tried changing my username...had to restore the system since it fully deleted it after letting me change it. I gave up Googling for the remote desktop and decided to test with the Windows Remote Desktop app in the Store to see if it was the iOS version. I got the store error and went on a tangent trying to resolve that. Lo and behold, I discovered the fix here that resolved both issues:
In the end, the simple solution is to make the account a local account. You'll have to sign back into your Microsoft account again, but RD and the Store both work great!