Thanks Zotto for your answer.
I had considered Ciborinia but rejected it as I never found any sclerotia  (I did search for it in more than 20 apos) 
Of course C. capillipes has close  features with my collection , including spore size, one nucleus in 8-spored asci, asci size , croziers, lack of crystals , lack of VBs in paraphyses. 
But there are also differences above the lack of sclerotia : 
- I found no 4-spored asci 
- C. capillipes seems to have been found in rather wet zones and in May or June as this one is an autumn find on dry calcareous soil (The Chizé forest, which you already visited -:) 
- The substrate is also different . I am now convinced that the substrate here is Carpinus . New founds this afternoon revealed apos developping on old Carpinus catkins (several times) as other seem to be on very small twigs pieces . As I could not find any apo under Fagus or Quercus litter, my feeling is that this could be actually linked to Carpinus , and I did find it everyxhere I searched it under Carpinus, including at home where I had planted a couple of trees 40 years ago ! Because of its growing under the litter, it can easily be overlooked. 
I attach new pics to show  sites and collections. 
Michel

I checked my database for Carpinus leaves and I did not find any Sclerotiniaceae with such a spore size. But the Carpinus seeds I cannot recognise. I see sclerotium-lime structures which might be deformed seeds?

I was actually looking for Carpinus seeds when I discovered this species as I had found a couple of apos of Hymenoscyphus carpini on a seed. But none of the Ciboria/Ciborinia stipe was on seeds . 
Nevertherless I am pretty sure about old Carpinus catkins as a major substrate , which my pics should show but I admit that they are not of great quality
Michel

Yes Zotto. 

I will ask for molecular analysis. Do the difference between Ciboria and Ciborinia appear clearly from sequences ? 

Michel

Ciboria is genetically very heterogeneous but I am not sure that we need to split the genus in a strong way. Ciborinia is now Scleromitrula, because it forms a compact clade with S. shiraiana, so yes, it is recognisable.