When connecting to RDP sessions from RDM on a PC with 4K resolution, for some (but not all) of the connections the resolution is such that the text is basically unreadable on the remote server. I remember this issue with W7/2K8 but this is a Windows 10 box running RDM connecting to a Windows 2016 Server.
I suspect that this issue might be caused by the HDPI setting. For this reason, could you please consult the following help topic and try both solutions?
Doing so should solve this issue: https://help.remotedesktopmanager.com/troubleshooting_hdpiscalingissues.htm