Activation link error

I am using Ultimate Member on my WordPress site.

I have a live site and a test site that are currently 100% identical and both WordPress and Ultimate Member are updated to the most recent version.

Everything works fine on my testserver, but on the live site users get this message when they click the activation link in the activation mail: “The activation link has expired or already been used”.

The two sites are using different webhosts.
My test site uses http, and the live site uses https so I’m guessing the problem is caused by some security measures?

The page I need help with: [log in to see the link]