Help! Repeat error in VShell logs - Connection from an IP rejected due to filter err

[dverbern]

I require some help - I am using VShell x64 v4.04.
For the past few days in the VShell logs, I have seen the following repeat error, which redacted is:

VShellSSH2 conn {SomeNumbers} IP address xxx.xxx.xxx.xxx {SomeNumbers} - - - 0 0 0 0 {My SFTP Server IP} 22 "{SomeNumbers}: Connection from 13.70.136.174 rejected due to filter error: Hostname lookup failed: host not found"

The IP xxx.xxx.xxx.xxx is listed in the Connection Filter, near the top of the list in terms of precedence, as:

Action: Allow
Source: xxx.xxx.xxx.xxx

If I enter that IP in the Test filter and click the Test button:

Result: Access denied. Reverse IP lookup failed.

It is entirely possible I am doing something wrong in my administration of this system. I'm running VShell on a Windows Server 2008 R2 VM, which sits in a DMZ, but refers DNS resolution requests to a series of domain controllers.

I don't know what the "Test" button in VShell does behind the scenes, but I'd guess it is something to do with host lookups, because when I do a NSLOOKUP of a IP or host that is allowed and passes the "Test" button, it returns a valid host, but conversely when I enter an IP or hostname that fails said Test, the NSLOOKUP likewise fails to return a host and instead returns:

nslookup xxx.xxx.xxx.xxx
Server: DomainController.domain.com
Address: {DomainControllerIP}

*** DomainController.domain.com can't find xxx.xxx.xxx.xxx: Non-existent domain