I get the same result when using Chrome instead of Firefox.
I can also make either video work – as long as it is the only video in the raceQs folder.
Also, the Video Time Offset persists across different races, meaning that different races (on the same day) cannot have different offsets. This actually makes some sense, but for some reason the offset is different for the different races. Perhaps this has something to do with how the GPS data is segmented into the different races?