If your Stripe 'Add Billing Method' won't load, please check on the following:
1. Most of the time this has something to do with the mod_security setting of your site. Unfortunately it is not possible for us to know the specific rule number which triggers the issue, we would suggest you speak with your hosting on this matter. They should know what necessary measures that should be taken for this to be fixed.
2. Check if one of your 3rd Party Plugins or your active Theme is causing the issue. We've known that the 'Insert Headers and Footers' plugin would cause this and it won't be fixed by disabling the styles and scripts under OptimizePress >> Support >> Disable Styles And Scripts. For this, we would suggest contacting the plugin author since we cannot help with issues caused by third-party scripts and plugins, the plugin author is the best person to help with the said issue as they know their code and script better than we do. Or, you can opt to find an alternative plugin/theme instead.
If the above suggestions don't help, please do contact our support team. We'd be happy to assist you further.