I checked there server and viewed their certificate saw that it was lets encrypt, I remember others having problems with them to because they have their own root certificate.
I just now searched in the community forum (I remembered vaguely that it was something with my.openhab.org) and found this topic My.openhab.org is online // iOS app doesn't show sitemap -> Solution: Java update!
Maybe @Dojokun is also helped by this post My.openhab.org is online // iOS app doesn't show sitemap -> Solution: Java update!