iOS CardDAV account cannot connect to NC - SSL error

recently I installed NC (15.0.0-1) on Arch Linux on a Raspberry 3 and got it running after a few days of configuration orgies.
For using SSL I created a CA cert with openssl and a self signed server certificate for the raspi (private IP address, no official DNS name) which I imported both together including a signing cert (don’t know how this exactly works) into iOS. I am able to access the NC Gui on my Raspberry via Safari using https on an iPad.
Trying to access via a CardDAV account in iOS to sync contacts leads me always to an SSL error message and it asks me to switch to http plain, which is strange, because https works in in the Browser on the same device like it should.
From a Linux client running Thunderbird with the CardBook app I am able to sync contacts sucessfully, so the CardDav sync seems to work in general.
In the apache logs I don’t see any unusual events.
To me it appears like an iOS SSL issue with my apache SSL config or maybe wrong parameters of my certificate which iOS doesn’t like.

Any ideas or similar experiences?

thanks in advance and cheers