Hi Václav,
Nice to see your photo and I’m sure it’s the first of many successful crops to come!
It looks like it’s just some of the smaller pins that have dried out – probably after aborting (this means they stop growing part of the way during the growing process).
It’s quite normal for some of the small pins to stop growing, enabling others to grow bigger. I think you have the right conditions of humidity and temperature – the long stems and some of the aborted dried out mushrooms may have been caused by CO2 levels being too high at the start of fruiting (something we discussed in a Facebook message), but it looks like you have sorted this out as the caps spread out and grew properly in the end.
I suspect that the next bag(s) you put in that growing space will grow perfectly 😉