I've tested this on 3 machines, 2 of the 3 had older versions of Vixen installed and stopped working, the 3rd had a fresh install. All 3 are Windows 10 machines with all the latest windows patches, same security software (avast/defender), and the same network config.
I've tried un-installing on one of the machines that wasn't working. I cleaned up the Vixen3 folder, re-installed and re-configured the display and it works again.... So something must be amiss with the core code too to make it stop working, or something that MS 'patched' and then patched again.
Now, when I copy the /My Documents/Vixen 3 folder from the non-working machine to the working machine, to copy all the sequences and fixtures and base config, because it's identical between the 2 machines, it doesn't work. The output controller has a red dot next to it. But when I then remove the controller and re-add it (same element, just patch to new controller ) and then try to open the original test sequence Vixen says Error Trying to open Sequence. But, I can re-create the sequence with this new controller and it works again. The DMX controller doesn't have this problem (DMX Pro on both machines with same port setting)
So it seems like either I'm doing something I'm not supposed to be that is breaking things (copying the Vixen 3 folder from machine to machine), or there is a bug in the core code that stops it from working when this is done.