ssh_rsa_verify: RSA modulus too small: 512 < minimum 768 bits key_verify failed for server

For me it looks like your ssh client is using a 512 bit key, and the FGT requires a key of 768 bit length or more. I've never seen this before as I'm using 1k bit keys.

Try to use a different ssh client, like Putty.

Or just re-create a key size  greater than 512K. IMHO nobody serious with security, should be using a key-pair less than 1024K. Once again , just  IMHO.

@emnoc:

key length is not measured in "K" these days...maybe in the near future

oops yeap that should 512 bits and not bytes.