Update still results in the same error. To be clear, after running 6.6 I reverted both files back to 6.5. That's when the errors started. Then I tried 6.6 again, but now the errors won't go away even with 6.6. To get a functioning Viewer again, I had to rename the config_4.xml file and let the Viewer create a new one (with the default settings and no key file).
So 6.6 changes the config file to make it incompatible with 6.5. And running 6.5 afterward corrupts something in the config file to make it incompatible with 6.6.
FYI, no details show for any of the Hosts in the 6.6 version of the Viewer. Looking at the properties of the Host I can see the Internet ID, etc., but they don't display in the columns of the Viewer when the "Details" view is selected.