Hey formulaic, thanks for reaching out, welcome to forums and I am sorry to hear about the issue you ran into.
To correct this situation, I would like to suggest that you follow the steps below (if these are not the steps you've already tried):
Firstly, please make sure you're launching Steam and Uplay with admin privileges, by following
these steps.
Then, please, navigate to the Ubisoft Game Launcher "cache" folder, by default located here: C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\cache
Open the cache folder and move the "ownership" folder to a safe location.
Once you have backed up that folder, delete the "cache" folder.
After deleting the cache folder, please verify the game files within Steam:
- Open Steam
- Right-click the affected game in your Steam library and select Properties from the drop-down
- Select the Local files tab and click the Verify integrity of game cache... button
- Steam will verify the game's files. This process may take several minutes
- Once the process is completed, the Check Window will automatically exit
If an issue is found, you will be given the option to Repair, click Repair. After repairing the issue, please verify the game again until no issues are found.
When this has been completed, try launching the game again.
Keep us updated and should you have any questions, let us know!