In the meantime, Walter Jaklitsch identified this fungus from my photos as the anamorph of a Diaporthe. In that genus the anamorphs are less diverse than the teleomorphs so that Walter prefers not to name this species from the anamorph alone. Actually, at least 5 Diaporthe species have been described on Acer. At the collection site there was Acer platanoides (natural) and A. pseudoplatanus (introduced), so one of these two Acer species must have been the host of this fungus.

According to Walter, a pseudostroma in the wood forming a so-called ventral zone (= black line in section) - as seen in our collection - is characteristic of many Diaporthe species, and is of course also found beneath the perithecia.

cheers
Zotto