Same problem.
Tor Browser 13.0.4
Windows 7
“html5: Video file not found” or just does not start.
This problem confirmed in KVC Player, JW Player and Fluid Player. Maybe in some others. Short videos (<10 minutes) sometimes work. Longer - no.
In Tor Browser 12.5.3 videos playing normally. If you do not open more than 9 browser tabs or do not start a video longer than 2 hours, of course. Otherwise the same error.