Hi Andreas,
H. armeniacus (= ochraceus), in shades of orange, is known to develop mostly on Russulales and to often completely destroy its host before spreading on surrounding litter, even at base of living trunks. Your spores are a bit too small to fit this species but they may be immature. See Rogerson & Samuels 1994, Mycologia, 86: 839-866.
Good luck!
Jacques

Hello Andreas,
 
wasn't there some corticioid fungus nearby? I had this Hypomyces, tentatively ID'd as H. corticiicola, growing on a piece of bark partly buried in soil, with Sebacina cf. incrustans growing on the lower side of the bark. Spore size looks similar. 

Viktorie 

I've seen perithecium wall discoloring in KOH to yellow to orange-yellow. I the protologue it is described as either turning bright yellow or not reacting at all. I didn't test LA and don't have the specimen anymore.

Hello together,
as it is sometimes coincidence, yesterday I found this Hypomyces on the ground in a wood I quite often visit. I never found this before, and it looks to me as it should be the same than Andreas fungus. My specimen is quite premature, only some asci contain ripe spores, and I found not a single spore outside the ascus.
The macroscopic appearance to me was like thrown-away peels of tangerine, and I had to look with the lens (in the dark forest) to see the perithecia and to regard it as fungus.
About the ecology: the forest round the specimen (not directly where it grew, unfortunately, but few meters away) was full of decaying and decayed remains of Russula specimens, what is also proved by the very common growth of Asterophora parasitica. I did not notice a corticioid fungus and the habitat did not look as if this would be very likely. As I also find the pictures googled for H. armeniacus fitting well I consider my specimen to be this species.
KOH did not lead to a notable discoloring of the perithecia(l wall) neither macroscopic (stayed the same bright orange) nor microscopical.
Yours, Lothar