No you don't need SSL when using Paypal Express.
The response from paypal says:
Security errorSecurity header is not valid
this means there is a problem with the API Credentials being used.
it may be that you have not accepted the billing agreement after creating the API Credentials.
It usually happens if the Live API Credential are being passed to the sandbox or vice versa.
it could also be just a bad set of credentials, you may want to try to regenerate them.