DNSmanager can be used both as a master and slave DNS-server. From the master server, DNSmanager receives Information about domain zones for which it is used as a slave server.
In Statistics→ Problem domains you can find a list of domain names. These are slave domains that could not get information from the master during the last 24 hours. Very often such issues occur because the domain is not active (the user account associated with this domain is suspended). If the domain is not updated for a long time (for more than a month), it probably contains outdated information. You can delete the domain from the server, as numerous problem domains make it difficult for normal domains to update.