Can you describe the structure of excipulum?

Of course,

outer layer of receptacle from textura globosa, cells with thickened walles, yellowish, end cells at the edge clavate.

Stipe has excipulum from textura porrecta, cells brown, slightly rough from incrusting pigment, with branching hyphoid hairs.

In my database I find on R. chamaemorus

Rutstroemia chamaemori
Ciboria latipes 
Sclerotinia tetraspora

all with a similar spore size. 

Did you compare all three? I do not know either of them but I suspect you are right with Sclerotinia tetraspora, because latipes does not fit and R. cham. is on leaves.

But I wonder about the excipulum. Your photo shows only the cortex of very small globose cells. Are they larger inside?

Zotto

Hello, Zotto.

The species has distinct scletorium (not stromatized tissues or black stroma), which points to the genus Sclerotinia.

I have seen Rutstroemia chamaemori on this plant (likely, but spores were curved which is not mentioned in descriptions) and showed it here before. http://www.ascofrance.com/search_forum/24922

Ciboria latipes - i have not met, but according to the key in Nordic Macromycetes (2000) it should have stromatized host tissues instead of a distinct sclerotium. Is it right?

I have microscopied excipulum once more, it is made from such small cells in upper part, no larger cells inside. Most inner tissue (medulla) made from interwoven hyphae. Cells become some larger at the base of receptacle. Probably the apothecia were also young, as many asci were underdeveloped.

Oh, I overlooked the small black sclerotium. Did you try a section of it to exclude that it contains some host tissue? This was formerly a basis to define Ciborinia.

The VBs in the paraphyses would point to a Botryotinia, by the way.