Symptoms
The customer is trying to add a new A-record (for the domain with hosting, thus with system A-record), but it is not possible:
There is a A record xxxxx (ID xxxx) already.
Cause
By default OA does not allow creating A-records when there is a system one. However this behaviour can be changed by modifying the system property Prohibit duplicate DNS records
in System > Settings > System Properties (set it to Disabled).