Windows 10-64 Pro - DO 12.1 x64 b.6089 upgraded recently from 11.19. I am receiving an error when I double-click to view an image in the standalone viewer. 
I've set preferences to "Use internal picture viewer for all recognized pictures." I have never had ACDSee installed. Double clicking d8viewer.exe directly from the install directory produces no results. I'd appreciate any help you have to offer for solving or troubleshooting the error. Thanks Bruce
That isn't the standalone viewer, it's something trying to run ACDSee, which is not part of the default Opus configuration either.
Whose configuration did you import? 
Please tell them to fix this in the config they're giving to people, and preferably also urge them not to encourage people to install a whole custom config over the top of the default one, as it causes problems like this.
To fix the problem, go to Settings > File Types > Recognised Images > Events > Left Double-Click and set it to run Show instead of ACDSee.
A couple of related threads in case the extra info is useful:
Thanks for the help Leo, point well taken about using an imported config. Shortcuts can lead to unexpected consequences. 
Bruce