five times in the list of webhooks in the Stripe dashboard, with four of these actually tracking activity. Does that make sense?
Yep, this is definitely is the cause of PHP errors. Also looks like Stripe changed error handling in their SDK, this will be addressed in the upcoming version of Subscriptions add-on.
Anyway, you need to delete all plugin webhooks manually from your Stripe Dashboard that look like https://(domain)/?asp_hook=XXX (both Live and Test). Then create those from Subscriptions tab of Stripe Payments settings, let the signing secrets stay there as well. Then check your Stripe Dashboard to ensure only one of those created for both Live and Test modes.
Let me know if PHP errors are gone after that.