SSH Public Key Format
Solution 1:
use ssh-keygen -i to convert SSH2-compatible format to OpenSSH compatible format.
from man ssh-keygen:
-i This option will read an unencrypted private (or public) key file in SSH2-compatible format and print an OpenSSH compatible private (or public) key to stdout. ssh-keygen also reads the RFC 4716 SSH Public Key File Format. This option allows importing keys from several commercial SSH implementations.
Solution 2:
This is the complete, correct answer:
ssh-keygen -i -m PKCS8 -f public-key.pem