Dear BSA,
I took a careful look at the picture of the snubber you posted above and I noticed that, by comparing the proportions on screen of the two picture, the gap in your example was smaller than mine.
The size of the gap on my bike was a bit smaller than 1 mm, which seemed small enough to prevent the belt from going up to the point it would slip. But I gave a try and reduced it as much as I could, to the point that only a sheet of paper could fit between the snubber and the belt.
Guess what ?
Next day I took my bike to go to work and voila: no more tooth jump, at all, even when pushing very hard on the pedals.
I’m back in business 
That’s so great. I feel really good now that I can enjoy a fast Strida.
I still have two issues, first a small one: some noises coming from the rear wheel, I will probably need to adjust the brake again or put some oil or grease between the freewheel cog and the frame, second is that annoying issue with the wheels rubbing against each other when the bike is folded, and for this by seeing the system from our Korean friend, I will probably have to be creative.
Thanks for your help and support, it really helped me fix my problem.