I also have this same issue.
I had the previous release installed and the plugin was working fine (although I kept getting 4 or 5 emails saying ‘Online ticket sales for are now closed) and then a couple of days ago I got the ‘WP HTTP Failed to contact the payment gateway: 403’ emails.
I updated the plugin to the current release but I still have the same issues.
I have currently stopped using the plugin and hope that the issue can be solved by the developer. It is a fantastic plugin.
Woody