Unable to access plugin configuration or deactivation

I recently got affected by the redirection to spam issue: https://wordpress.org/support/topic/redirecting-to-spam/
​I followed the instructions in order to solve this. First I updated the plugin. The problem arrived in the next step: I am supposed to get to the configuration options. But I am unable to do this, because I am stuck in the “designer_welcome_page” form, where I have to chose if I provide permissions to opt in for Email Notifications. The options given are in two buttons: “Skip & continue” or “Opt-in & continue”. Neither of the two buttons is working, so I am stuck here. This is a malfunction of the plugin. I tried to deactivate the plugin but another form appears asking for the deactivation reason, and again, the button doesnt do anything. In the bottom status line I can see the sentence: “javascript:(0)” when I push those buttons.​
​This brokes my website completely and I really need a solution.
​Please help.

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