Storage/SFTP returning "Malformed OpenSSH private key"

[bekarau]

Hi,
Trying to set up a backup from an instance (2.5.289) by publishing configuration in the STORAGE/SFTP section. After pasting and typing the various items in, I activated/applied the settings and was shown a small red error stating "Malfourned OpenSSH private key". This appears to be generated by the ssh2 modules being used by the ssh2-promise wrapper module.

I have tried various different types and formats, and even attempted to revert to just using password authentication, but the error persists.

I don't have access to the system logs to get further details (only info available is what's via the web admin interface).

Tried:

-----BEGIN OPENSSH PRIVATE KEY-----
...
-----END OPENSSH PRIVATE KEY-----

-----BEGIN RSA PRIVATE KEY-----
...
-----END RSA PRIVATE KEY-----

Both as generated from ssh-keygen and manipulated to remove CR/LFs between the markers.

A quick peek at the ssh2-promise and the ssh2 modules on github suggests that the first block of data should have worked.

Any thoughts on what could be causing the issue?