I face the same issue I solved passing the credential to the format that looker needs!

Try using this python code and use the new key

from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.backends import default_backend

input_path = "client-key.pem"
output_path = "client-key-pkcs8.pem"

# read original key (PKCS#1)
with open(input_path, "rb") as key_file:
    private_key = serialization.load_pem_private_key(
        key_file.read(),
        password=None,
        backend=default_backend()
    )
# save in format (PKCS#8)
with open(output_path, "wb") as out_file:
    out_file.write(
        private_key.private_bytes(
            encoding=serialization.Encoding.PEM,
            format=serialization.PrivateFormat.PKCS8,
            encryption_algorithm=serialization.NoEncryption()
        )
    )
output_path