Adding https Subsite

Hello there,
I want to change one of my subsites from http to https.
My main domain is already https and now I activated a Lets Encrypt SSL certificate etc.
I removed the http version of the website in domain mapping. relogged in with the network adress and added the domain with https. But every time I add the adress and submit it, its saved as http and not as https.
When I log in to the network area and go to domain mapping, it is added as https. But on the subsite itself it is listed as http.
When I set up the Domain to force https - I get a too many redirects error.

I tried following:
1. change http://www.career-competence.at to https://www.career-competence.at with better search and replace plugin
2. I deactivated all plugins except domain mapping
3. I removed all redirects form htaccess file
4. I changed the siteurl and homeurl in /wp-admin/options.php to https://www.career-competence.at

But nothing helped... Do you know where the problem could be?