If you could not connect remote server by Remmina tool on Ubuntu server just following the steps,

cd ~/.freerdp/

# ls -al


total 16

drwx------  3 user user 4096 Aug 12 09:27 .

drwxr-xr-x 30 user user 4096 Aug 12 09:33 ..

drwx------  2 user user 4096 Apr  9 11:28 certs

-rw-rw-r--  1 user user  725 Aug 12 09:27 known_hosts
# vim known_hosts

or directory open it, 
 
# vim ~/.freerdp/known_hosts


192.168.1.35 df:04:a3:25:26:4b:4b:76:34:37:4d:86:06:c2:63:6a:e7:a9:e6

192.168.1.12 72:9e:c5:60:f9:a7:3f:7b:cf:a7:6b:d9:c1:a0:31:af:e4:76:79

192.168.1.19 df:77:75:15:70:0e:20:c2:57:be:cb:5a:a1:d8:f0:e8:5a:a8:ba

192.168.1.33 30:f9:7f:fe:c3:ab:14:a4:c2:81:31:10:5c:f9:a8:74:dd:45:7f


remove that specific IP address line and save it.

:wq!