Currently JasperServer when connecting to SFTP supports only BEING RSA PRIVATE KEY. This is limitation on JSch library level which is used by JRS when connecting. The enhancement request is to add support of keys that start with BEGIN OPENSSH PRIVATE KEY
Vadym Chepuriaiev
| Components | JasperReports Server, Scheduler |
https://stackoverflow.com/questions/53134212/invalid-privatekey-when-using-jsch
see here