Recent versions of OpenSSH create SSH keys in a new format. You can identify the new format when looking at the key and it starts with "BEGIN OPENSSH PRIVATE KEY" and not "BEGIN RSA PRIVATE KEY". Solid Explorer doesn't accept these private keys for SSH connections, so one must convert them to the old RSA PRIVATE KEY format.

It would be nice if Solidexplorer would accept these "new" keys out of the box.