Dear Marta,

from your pictures it makes impression to me that apothecia are pruinose and asci have amyloid structures in apical part - I think these features exclude Hymenelia. You may still check for paraphyses (they should be apically constricted at septa in Hymenelia - submoniliform). One of the options is that your sample will belong to a segregate of Lecanora, currently called Myriolecis or Polyozosia (on Italic web), at least one taxon/or?form from the Myriolecis agardhiana aggregate may form apothecia without thalline margin, it was described as Lecanora lecidella Poelt, and on Italics web this name is treated within synonymy (with ?) of Myriolecis/Polyozosia agardhiana subsp. sapaudica. The complex is nomenclaturally and taxonomically complicated and needs revision and I am not familiar with it at all, nor sure whether your sample is Myriolecis/Polyozosia at all, just an idea. Probably Claude Roux knows these lichens quite well. 

All the best

Zdenek

Dear Zdenek Palice
Thank you very much for your comment. I have studied the paraphyses in lactophenol blue and they are not submoniliform. They are septate and sparsely branched.
After your comment, it occurred to me to use Italic 8.0 key 85 of 'Lecideoid crustose lichens' and I found Clauzadea immersa. I don't understand why I haven't used this key before!
I attach a photo of the paraphysis and the hymenium in melzer.
Thank you very much again.
All the best.

Marta.

Yes, Clauzadea immersa sounds like a good choise, it was likely just impression, indeed, the apothecia are pruinose, I was not able to see clearly any tube-structure in ascus tip before, but they seem to be visible (though not so clearly) in one of the new series of photos. the ascospores (if not too old) may have distinct perispore, according to monographer of the group (Meyer 2002, Sendtnera) the perispore is even ornamented in this species, but this is probably hard-to see inside asci

best wishes

Zdenek