task factory sftp - could not connect to the server using private key authentication

Using task factory 2020.18.1 with visual studio 2019, any target server version, but will be deployed to a sql 2016 server.

When trying to set up an sftp connection using authentication by key file, the test connection button shows the error

"Could not connect to the server using private key authentication. Error was ''"

The connection type is SSH FTP. If I switch to "SSH FTP LEGACY" the error is:

"The negotiation of key exchange method has failed".

The format of the key file originally provided to us by the host was putty. Using this putty file works in winSCP.

Using puttyGen I have converted the key file to both old format and new format alternatives (file header "-----BEGIN RSA PRIVATE KEY-----" or "-----BEGIN OPENSSH PRIVATE KEY-----").

The converted file is successfully used by openSSH to make a connnection.

No version of the key file works with task factory. I understand it can't use the putty format (the error is different, it says it cannot open the file). But it also doesn't work with the converted file, in either format.

As you can see above, the detailed error message is an empty string, which is unhelpful.

Any ideas? Cheers.