So, I have found the bug now.
If I enter the DynDNS url in the array at the position “0”, so at the first position it works.
As soon as the domain is in another position than the first one, it is not recognized as trustworthy anymore. This has no influence on IP addresses.
Interestingly, I can not connect the Android app via DynDNS. Here I always get the message that the host can not be found.