continuous tls handshake

I have recently aquired a certificate from lets encrypt and enabled https on my wordpress site.
The site works fine, but my problem is, that the browser seems to make the tls handshake over and over again when users are logged in. That means it seems to load again and again.
This stops when I deactivate the Ultimate Member plugin, therefore I thinks the problem is connected to this plugin.
I have updated the plugin, but it didn’t help.
Any other suggestions to, what I can try?

Greetings from Lars

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