I have recently upgraded from win 8.1 to Win 10. Since the upgrade RDP wouldn't work correctly.
I couldn't use a recently used connection. Instead I had to open a new window each time. Today it stopped working completely.
If I tried to run mstsc.exe directly it would appear in task manager for less than a second before dissapearing.
Edit: Issue was traced to "custom scale factors" in display settings. See answer below.
Issue was traced to "custom scale factors" in display settings.
This fix on technet worked for me. I am reposting here for others. (since most links in my search results were way off the mark)
I would add that if you have it pinned to the task bar it would be worth unpinning and re-pinning it also.