Please update the original document to include this information, it may save people hours of troubleshooting erorrs with privatekeys.
To convert the .key file to a compatible RSA PEM file you may need to add
-traditional to the end of the command.
openssl rsa -in currentprivatekey.key -out newprivatekey-with-rsa.pem -traditional
Newer versions of OpenSSL say BEGIN PRIVATE KEY because they contain the private key + an OID that identifies the key type (this is known as PKCS8 format). To get the old style key (known as either PKCS1 or traditional OpenSSL format) you can do this:
openssl rsa -in server.key -out server_new.key
If you are using OpenSSL 3, you need to add -traditional