I experienced few times a gross layer of foam when fermentation finishes. But the beer was fine, no symptoms of contamination neither off-flavors. I never measure the depth of foam but it can have about 2 to 3 cm (among 1 inch).
What can cause this ?
Is that explanation possible? : One of the explanations I found around the internet is that this foam is composed by dead yeasts, died by "hydration shock" when it is directly spread over the wort instead of pure water. This is strange to me, because although I use dry yeast spread directly over wort (at equivalent temperature), I think it would be necessary lots and lots of dead yeasts which isn't all of them within yeast packet. So, they would need to reproduce and dead yeasts don't reproduce, so the explanation about the "hydration shock" is very strange, which makes me think that if the foam is composed by dead yeasts the yeasts died after reproduction and not when spread over the wort.