Then my question is on the client sever, what would trigger the SFTP server not offering the key authentication methods?
It's the server that controls it.
Anyway, if you want us to check, please post log files from both computers.
Yes, I understand this is from server end response, however the wired thing is that if we using the same key, the same username and from the same network on another server, it works, logged in straight. as the script must run from that application server to upload files to our customers side, we can't rebuilt a new server, and also the server end at customer side, the log I won't be able to get.
I attached our end debug 2 log, if we can file what from our end would trigger that behaviour, then we can change from our end