A domain named example.com cannot be synchronized to name servers, the following error can be found in
/var/log/messages on the name servers:
May 19 14:05:02 ns1 named: transfer of 'example.com/IN' from 203.0.113.2#53: failed while receiving responses: CNAME and other data
As it is stated in RFC 1912 in section 2.4:
A CNAME record is not allowed to coexist with any other data. In other words, if suzy.podunk.xx is an alias for sue.podunk.xx, you can't also have an MX record for suzy.podunk.edu, or an A record, or even a TXT record.
Please check the DNS zone of example.com for records for aliases and remove either them or the CNAME record(s) in question.