I had a similar issue with Plesk Obsidian and Github and I manage to resolve it with the help of a support agent from Plesk (through teamviewer). You basically have to delete or rename the id_rsa, id_rsa.pub and known_hosts files from the .ssh folder and recopy any ssh keys to your github...
Anyway, issue is solved by deleting id_rsa, id_rsa.pub and known_hosts file from the .ssh folder. You will have to re-copy the ssh key and clone the repo again.
So, with SSH port 22 and the same settings on a Plesk Onyx server this is working no problem. It might be an issue with Plesk Obsidian and the Git extension.
I am using this method mentioned here: Is it possible to connect to private Github repositories using Plesk Git extension?, but I am unable to connect. I always get the following error:
Public key for the server at 'github.com' is already known in...