Puttygen export openssh key

I'm having trouble configuring SFTP with Prepros 6. I’m using the exact same SFTP configuration as I did with Prepros 5, which always worked. SFTP, same Remote Path, same Host, same Port (22), same username, same Private Key. I copy the contents of a .pem file into the Private Key field.

The first time I try to upload a file from the FTP page, I get the error

An error occurred while uploading file via FTP. Cannot parse private key: unsupported key format.

On subsequent attempts, there is no error message but the spinning working icon appears but never goes away and the file is never transferred. I have to restart Prepros to get the error message again.