I finally found it.

There is a .dll in the Windows > System32 directory called atig6pxx.dll. By changing the name of this .dll to atig6pxx.dll.bak the error goes away.

Apparently this problem has been around for some time. The instructions I found were from 2016. After doing the above, I also checked to see if this file gets rebuilt during startup. It does not.
Note that this problem is still in the very latest version of Windows (Spring Update - 1803). If you run across it, xLights will not start until you get past it.
