WinSCP 6.5: Intermittent Connection Issues

I am intermittently receiving the following error:

Remote side sent SSH2_MSG_EXT_INFO not either preceded by NEWKEYS or followed by USERAUTH_SUCCESS

I have previously posted a topic that discussed this very issue https://winscp.net/forum/viewtopic.php?t=34390.
In my original post I claimed that I recently added a ED25519 Hostkey to my SFTP Server. I do not know how relevant this is to my issue; it might just be a coincidence?!?!
Furthermore, in my original post I also claimed that the problem now appears to be working without issue. I'm not convinced this is the case.

I had a new requirement to automatically upload some files onto our SFTP Server from a new server. Now that I've scripted it I'm getting this error again. I am also using the WinSCP GUI Client and I experience this issue when using that too.

Again, if I use the WinSCP GUI Client, if I keep pressing the 'Reconnect' button eventually I get connected successfully. Sometimes it just connects, sometimes I have to press the "Reconnect" button once or twice and on other occasions I have to press the "Reconnect" button loads of times!

I am using the WinSCP v6.5 and my SFTP Server is using a commercial product named Tectia Server v6.6.2.

Using the WinSCP Client v6.5 I just tried to connect to my SFTP Server and I had to press the "Reconnect" button TWENTY times. On the 20th time I connected successfully. I have attached this log file to this ticket. I have obfuscated some data including usernames and server names.

If I use the OpenSSH SFTP Client that comes with Windows (version 9.5p1) no matter how many times I try, I cannot get it to fail!

Any help to identify and locate the problem would be much appreciated.

Many thanks,

Lee