metiulekm , 5 hours ago

It seems OP wanted to pass the file name to -k, but this parameter takes the password itself and not a filename:

<span style="color:#323232;">       -k password
</span><span style="color:#323232;">           The password to derive the key from. This is for compatibility with previous versions of OpenSSL. Superseded by the -pass argument.
</span>

So, as I understand, the password would be not the first line of /etc/ssl/private/etcBackup.key, but the string /etc/ssl/private/etcBackup.key itself. It seems that -kfile /etc/ssl/private/etcBackup.key or -pass file:/etc/ssl/private/etcBackup.key is what OP wanted to use.