The LeadPages Connector plugin is compatible with OptimizePress, but we have found that if you have their 404 option active to capture 404 pages, this interferes with or breaks our Optin form functionality.
This is because when your Optin forms are submitted inside OptimizePress they pass the information through a page URL Which doesn’t actually exist on the server (/Process-optin-form) and therefore this triggers the 404
If you turn off the 404 functionality inside the LeadPages plugin, your optin forms should work fine