Hey Iris, hey Thomas,
did you get around to solving this issue? I had the same issue with a file called “Werbung.wav” or “Werbung.mp3”. And as you, Iris, I tried a lot of different things. I have to add though that the error in my case only occured in Mozilla Firefox with some of the persons testing it. So it was a specific Mozilla+certain persons and not systems problem. In the end, I realized that the word “Werbung” in “Werbung.wav” [advertisement in engl.] apparently triggered a specific adblocker or an add on in Mozilla. So I simply changed the name of the file and then resynchronized. Now it works… (And no - this is not an April fools joke 
)
All the best,
Alex