Can't connect to SFTP in TMD hosting

I have contacted TMD hosting support and they also reproduced the error.
They are providing the following login details:
rotocol SFTP - SSH File Transfer Protocol. On the port, please set <removed> and enter the following information below:

Host: <removed>
Logon type: Normal
Username: ftp username
Password: ftp password

And we keep receiving the following error:
[10:11:17] FlashFXP 5.4.0 (build 3960)
[10:11:17] Support Forums https://oss.azurewebsites.net/forum/
[10:11:17]
[10:11:17] Winsock 2.2 -- OpenSSL 1.1.0e 16 Feb 2017
[10:16:21] [R] Connecting to mexiadesign -> DNS=<removed> IP=<removed> PORT=<removed>
[10:16:21] [R] Connected to mexiadesign
[10:16:21] [R] Host key algorithm ecdsa-sha2-nistp256, size 192 bits.
[10:16:21] [R] Fingerprint (SHA256): Vjlhmk99+CoKnNhZCCarEt0LoiZGDj09mxMudj/ZDtc=
[10:16:21] [R] Key exchange: curve25519-sha256@libssh.org. Session encryption: aes256-gcm@openssh.com, MAC: <implicit>, compression: none.
[10:16:22] [R] Connection failed


And they answered this:
Nick Spencer
Staff

12/03/2017 09:49

Hello,

Thank you for your patience.

We have downloaded FlashFXP 5.4.0 (build 3960) and we were unable to connect as well using SFTP connection. We are not able to provide you with any information regarding the error, further investigation must be carried with their support team. We recommend using FileZilla for e.g:

https://filezilla-project.org/

Why is this happening? Can you please fix it?

thanks!

Hello,

Thank you for reporting this issue.

I am not entirely sure if this is a compatibility issue with the SFTP client or server but I was able to reproduce the problem

Do you happen to know what SSH/SFTP server software they are using?

I will investigate this issue further and provide you with any new information I find.

After basic testing I found that by disabling the aes-gcm ciphers allows FlashFXP to connect to the site.

Open the Site Manager and select the Site profile.

Then click the SFTP tab, under Encryption select Use Custom Settings and uncheck aes256-gcm@openssh.com and aes128-gcm@openssh.com and click the Apply button to save the changes.

This is great news, I will ask them what SSH/SFTP software they use so I can provide you that information. Thanks!

TMD answered this:

Hosting service is using CentOS with the OpenSSH service.

I have attempted to reproduce this problem locally on a VM using CentOS and the same version of OpenSSH server but have been unsuccessful.

So I have been using the IP/PORT of your SFTP server for testing, if this is a problem please let me know.

There is clearly a compatibility issue with AES GCM ciphers on this server.

I decided to test the latest version of FileZilla and the same issue was observed. I suspect that maybe your version of FileZilla did not include the AES GCM cipher or it is/was already disabled.

Going forward the simple solution would be the same as my initial suggestion and that is to disable the AES GCM ciphers.

Since I cannot reproduce the issue with a similar CentOS / OpenSSH VM I suspect that there is something specific to the configuration or setup on TMD hosting.