I dont know why this happens,
but on some of my Citrix Servers you sometimes cannot connect via RDP
and guess what, a reboot doesn’t fix it.
Well, this handy little script will help solve your issue, not only with RDP, but also with ICA
RDP / ICA Reset Script
I dont remember exactly where I got this script but its been in my toolbox for a while, credit to the originator whomever you may be