Certificate issues with Iphone

While I can access my nextcloud server from Linux, android and mac, when I try to connect with iphone I get:

The certificate for this server seems to have changed
Do you considerā€¦

If I say yes , it goes away until I try do something else with nextcloud. It does not transfer any files.
I am running nextcloud 24.0.2 and ios 16.0.2

Does anyone have an idea?
Thanks, andi