Latest Joomla versions 3.9.x introduce a new system plugin - privacyconsent (more information here).

When enabled it completely breaks Smart Subscriptions registration and edit profile flow. New Joomla plugin intercepts all requests except those coming from core Users component to make sure that site pages cannot be accesses before Joomla users agree with the privacy policy, and if user consent was not given it redirects to edit profile view.

Smart Subscriptions component uses its own controller to register users and update their profiles (custom controller is required to support payment plugins), which in combination with privacyconsent plugin enabled results in infinite redirect.

In the latest Smart Subscriptions 1.3.8 we introduce a new system plugin (Smart Subscriptions J3.9) which provides a workaround. This plugin doesn't require configuration, just enable it if you are planning to use Smart Subscriptions on Joomla sites with privacyconsent plugin enabled.