Connection Error in Guacamole: The remote desktop server is currently unavailable. If the problem persists, please notify your system administrator

I am trying to setup up guacamole in a Digital Ocean Droplet (Ubuntu 18.04). I followed the steps provided in https://computingforgeeks.com/install-and-use-guacamole-on-ubuntu/ to setup guacamole and used Postgresql to authenticate guacamole by following the instructions provided in https://guacamole.apache.org/doc/gug/jdbc-auth.html#idm46227496294336.

The installation got over and I am able to access the webpage at http://droplet-ip:8080/guacamole, but when I try to connect to a remote machine over RDP I get a connection error stating 'The remote desktop server is currently unavailable. If the problem persists, please notify your system administrator, or check your system logs.'

I have checked the login credentials of the remote device, it's hostip and RDP port number, everything is correct. I am able to login to the machine through Remote Desktop Connection in Windows. I can also login to the same remote machine with same credentials in a perfectly working guacamole setup in another digitalocean droplet.

I have also tried this by installing guacamole using docker by following instructions provided in https://wiki.networksecuritytoolkit.org/index.php/HowTo_Setup_Guacamole, but still face the same problem. What am I doing wrong? I would be happy if someone could help me solve this problem

I was finally able to figure out why I was not able to connect to a remote device in Guacamole.

1. My Digital Ocean Linux droplets had freeRDP already installed. But Guacamole Server 1.3.0 works on freeRDP2. I had to make Guacamole send requests through freeRDP2.
2. I have enabled SFTP in the connection settings. But somehow the OpenSSH was corrupted in the remote machine resulting in connection error. So, I disabled SFTP. I think guacamole tries to establish RDP and SFTP connection in the very beginning, so even if one of the protocols fail, connection cannot be established. I am not proficient with guacamole so not sure with this point.

After resolving these problems, guacamole was able to send connection request to the remote machine. I checked the status using netstat and the status was SYN_SENT, but there was no response from the remote server. The problem was Firewall.

I allowed the ports for RDP in windows firewall, but the remote machine was in a network which had external firewall. I added the Guacamole Server IP in allowed list for NAT forwarding in the firewall device and finally I was able to establish a connection with the remote machine.