Other Optin Form Errors
We have found that some caching plugins that have a cache lifespan greater than 2 hours can cause a failure in WP Nonce checks which is something to help with form validation. This could cause issues with any plugin that submits data via forms (not just OptimizePress).
If you get any errors that indicate an invalid form entry or similar errors, and you have a cache plugin such as WP Rocket or Siteground caching, please ensure that the caching plugin is set to expire the cache automatically every 2 hours or less to avoid any issues with form validation.
For further reading on what WP Nonce is, we suggest this article: