I receive this error when being redirected back from Paypal after a successful payment transaction:-
"We're sorry, but we could not fulfil your request for [/my webpage] on this server.
You do not have permission to access this server. Data may not be posted from offsite forms.
Your technical key is: [I have not written it here]
You can use this key to fix this problem yourself.
If you are unable to fix the problem , please contact [my email] and be sure to provide the support key.
If I deactivate the plugin, it works fine albeit without SSL.