Prosites paypal button just refreshes page

We are using Prosites on our network and the Paypal button just refreshes the page after selecting a level for purchase.

I've read through all the previous threads on this issue and can confirm that we have:
- Switched from live mode to sandbox and vice versa
- Confirmed we are using HTTP not directing to HTTPS
- Recopied API credentials and checked there are no trailing spaces or periods
- Requested new API credentials and entered them in as above
- Enabled IPN
- Updated to the latest version of Prosites

The only thing we can think of is that something broke in the move from our testing server (where it was working) to our live server.

We are on a time crunch on this, so a quick response would be most appreciated.