Thanks for the feedback, do you have any other Stripe plugins running at all or plugins that may unload assets?
You mentioned you have upgraded the API, just to confirm you upgraded the version of the API inside the Stripe dashboard?
The reason you are seeing this error is that the Stripe class that’s included in Paid Memberships Pro is not being instantiated or missing it seems. I haven’t been able to replicate this issue.
You also mentioned that your theme comes bundled with Paid Memberships Pro? Could you please ensure that the Paid Memberships Pro you have installed isn’t altered and has the following file in your installation:
* Stripe class: https://github.com/strangerstudios/paid-memberships-pro/blob/dev/classes/gateways/class.pmprogateway_stripe.php
* Stripe library: https://github.com/strangerstudios/paid-memberships-pro/tree/dev/includes/lib/Stripe