Converting putty private key to openssh format causing Invalid key length failure

I’ve a putty generated keypair file from which I try to save the private key in openssh format with:

puttygen -O private-openssh privat.ppk -o privat.pem

But when I try to use the private key for openssh publickey auth I always get the

debug1: load_identity_file: Skipping key /home/gerijani/.ssh/id_rsa: Invalid key length

I’ve tried to convert it with the windows GUI version into new openssh or ssh.com format. I’ve tried to convert with cli on Fedora system as well, but the same result. I’ve also tried to convert the existing RSA key to OPENSSH format with:

ssh-keygen -p -N "" -f priv_new.pem

Meanwhile if I try to login to the target node with putty, login is always successful.