Ronald, can you confirm what your primary/DNS server addresses are set to under Settings>General? Are you using a local DNS server or do you have at least one public in there? I'm wondering if you're getting intermittent cached results and its leading to a false positive. If you have just private ones here, could you try setting one of the following to see if this clears up?
Google DNS 1: 8.8.8.8
Google DNS 2: 8.8.4.4
CloudFlare DNS: 1.1.1.1
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com