OK, so maybe I have more than one problem...
I installed 3.6.2, the oldest version I had an install image for, and it launched without the python error. The camera didn't display though. So I removed the 3.6.2 install and also got rid of any environment variables pointing at different installations (PYTHONHOME and PATH both had references) and relaunched. Again, no python errors and no displayed image. (I'll likely have to tweak some python settings to allow the other packages that use it to have access to the versions they want.)
So, though python version mismatches were the most visible of the errors I have something else going on as well. I'm connecting to a DCS-910 using DNS on port 80, Auto Detect correctly decides the camera is a DCS-910, using video channel 1. The User Name and Password are correct. I can view the camera from the web interface, but in D-ViewCam is stays blank.