Ultimate Membership Pro - Error activating Mollie Payment Gateway

Hi there, is there anyone out there who’s been using the Ultimate Membership PRO plugin from Azzaroco? And in particular, trying to get the Mollie Payment Gateway activated. I’ve spent numerous tickets and hours to get this solved with Azzaroco, but they don’t respond for some time now.
Anyone facing the same problem? When having entered the API and attempt to activate the gateway. While all is done correctly UMP keeps popping up with the message that no payment gateway has been activated or properly set.

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