Hi Joseba,
this looks after Orbilia (aurantiorubra-group, possibly O. xanthoguttulata ?) One must press the spores properly from the ascus. Whether this is correct I also don't know. Zotto has there definitely the correct answer.
In addition I don't have microphotos unfortunately any more.
Greetings Peter.

Hola Joseba & Peter
indeed this is O. xanthuguttulata! The spores are visible inside the asci, especially the apical spore body (I put arrows). The basal spores are inverted, as one arrow indicates.
 
Typical are the ellipsoid-spathulate paraphysis apices partly tipped by a hyaline exudate cap and containing yellow oil drops which stain green in iodine as you properly show.

Did you have rain and the fungus was hydrated? Normally one finds them in the dry state on exposed substrate. This is the inner side of bark, of a branch I assume. Was it still attached as you are sure with Fagus? What are these whitish fruitbodies in rows?

I would be happy to know the collectionn data and coordinates. In northern Spain there is only one sample in Asturias and one in Navarra, in southern Spain there are more, from Cataluna to Andalucia, also two in Guadalajara in the centre. The substrate is very diverse and includes also conifers. Collections go up to Finland.

Zotto

Hola Zotto,  muchisimas gracias por tus comentarios  y la identificación.

Te respondo a tus preguntas:

1º Si llovió los dos dias anteriores y el tronco de unos 15 cm. de diametro estaba semienterrado,  cuando intenté cogerlo,  me quedé con la corteza en la mano.
2º creo que es Fagus,   ya que es el arbol predominante en la zona,  tambien hay algunos Corilus,   pero me parece que  son menos gruesos...  pongo una foto de la corteza,  por si tu puedes identificarla.
3º Los frutos blanquecinos a mi me parece que son Exidia thuretiana o Myxarium nucleatum,   pongo una foto.
4º te paso los datos y corrdenadas a tu correo


Saludos
Joseba