If you are connecting to a remote Remote Desktop Server and want to get the identity prompt back

Goto

HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Servers\

Find the key for the computer you have just trusted it could be an IP, hostname or fully qualified domain name

i.e. Something like

19.70.21.21

rds01.toggen.internal

So if the computer you were trying to bring back the Remote Desktop Connection prompt was 19.70.21.21 you would delete the key

HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Servers\19.70.21.21

or just the

HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Servers\19.70.21.21\CertHash value