Does just opening Ubisoft Connect independently of steam cause the freeze?
Ideally, some kind of error or other issue will show up in Event Viewer’s application logs but the nature if the issue may prohibit that.
Since Steam is launching Ubisoft Connect which is then launching the game executable, I’d be inclined to find out which specific step is causing the freeze (Ubisoft Connect process creation or Game process creation)
You can use Process Monitor https://learn.microsoft.com/en-us/sysinternals/downloads/procmon and sort by process creation time to hopefully catch what is being created and it may be visible on the screen after triggering the freeze.