Issue Master DNS not working anymore

[Erwin Fiten]

I'm using 12.5.30 Update #53
Operating System: Debian GNU/Linux 8 (jessie)
Kernel: Linux 3.16.0-4-amd64

I'm using PLESK as a master DNS server. This has worked for a long while.
But now I noticed it isn't syncing anymore.
The system is installed on VMWare on a corporat network
According to our Firewall guy, this port isn't blocked and there are request passing from the slave DNS server (Openprovider), I also asked there to check, and they tell me they are not able to read out the DNS data.

Is there a way I can check if my Plesk DNS server receives the request from the slave DNS ?
How can I check what's wrong?

Erwin

 

[Erwin Fiten]

The Error i'm getting when checking with the online tool from slave-DNS is : "Openprovider can not read your name server for the specified domain, and can not function as a slave name server."