DNS problems on core 138

After a clean install of 64bit core 138 there seems to be problems with DNS resolution.
Several ddns.net addresses can’t be resolved.
Trying to ping xxxxx.ddns.net gives ping: xxxxx.ddns.net: Name or service not known
Pinging no-ip.com gives the same.
But a ping to heise.de is fine.
Test on my notebook shows that all ddns.net addresses work fine.
I suspected Suricata, but stopping the service doesn’t make a difference.
Restarting unbound didn’t help either nor changing the DNS server from Telekom to Google.

Has someone encountered a similar problem?

Are you using Cloudflare?

No, it’s just a direct German Telekom VDSL connection.
I suspected the Telekom DNS servers, but they are fine.
No Cloudflare involved.