Hi Walraaf,
Before proceeding any further, make sure to add and verify all of the listed domains on your SSL certificate as described in this “Adding custom domain to your application” guide. You may be able to retrieve the unverified domain(s) in step 3 using your console’s Custom Domain Page. In fact, all of the domains listed on your certificate are required to be added to your project prior enabling SSL on them. Note that it may take some time before your added resource records in step 6 are put into effect.