Can you let us know how OPMS does Domain Analysis ? If the monitor is working fine most of the time, why would it get Nameserver error all of sudden ?
The agent uses standard DNS name resolution to perform Domain Analysis. To ensure correct behavior we do not rely on ISP's DNS server but run a DNS recursor on every ASM monitoring station - powerdns. See https://www.powerdns.com/recursor.html
for more information on this product.
It may be that the DNS Server did not respond in time or gave an empty response. You should look into your DNS/networking environment to determine anything more.