Hmm that's strange. Hopefully Dynadot staff will read this and check on their end. Is there an alternate name server you could use like ns1.imingo.net ? This one seems to be accepted.
> sorry for my poor english but what is the "pond" ?
The name server you are using is ns1104.imingo.net. We checked the server, and it is not a COM/NET registered name server. It is a child name server of ns1.imingo.net.
It works for vilasco.org because the central registry for ORG does not care that the server ns1104.imingo.net is not a COM/NET registered name server. DNS is able to find the IP address for vilasco.org through DNS delegation.
For the details of how this DNS delagation is working: http://www.dynadot.com/domain/ip.html?domain=vilasco.org&details=1
It does not work for fx-kent.com because the central registry for COM/NET knows that ns1104.imingo.net is not actually registered. (The COM/NET central registry is also in charge of all COM/NET registered name servers).
The COM/NET registry requires all COM/NET name servers used with a domain to be registered with them.
The solution is to register ns1104.imingo.net with the COM/NET registry. The owner of the domain imingo.net would have to do that. Or to use another name server for fx-kent.com.