Thanks a lot for your fast replys!
@mnordhoff was right, my secondary DNS Server was unavailable.
I’ve correct the records in the whois database and add it back to my namserver.
Now it is working! Thanks.
@jsha why my DNS Server (Bind 9.97) answered with SERVFAIL when one of my second Nameserver is missing is not really clear but now it is working. Both Nameservers are available.
I’ve not set a CAA Record.
Thanks a lot!
xabbu