The ascospores of P. convexella (= P. constellatio) are usually larger, i.e. 16–19 µm diam.
So probably another species.

Nicolas, thank you for your comments.  There seems to be some uncertainty about the spore size for P. convexella/P. constellatio:
Pfister (Synopsis of the Genus): 18-20 (as P. convexella)
FoS: 13-17 (as P. constellatio)
Dennis: 13-15 (as P. constellatio)
Beug: 13-18 (as P. convexella)
Seaver: 15-20 (as Lamprospora constellatio)

In Pfister's Synopsis, P. convexella and P. cinabarrina match in apothecia color, spore features (other than size) etc.  P. miltina is not described in full, referring to Rifai (1968) (which I do not have).  But I have seen reference to P. miltina elsewhere (Bioimages UK) which seem to align with my collection well:
Apothecia: 3-7mm diam.
Ascospores: spherical, hyaline, smooth,14-16µm diam., with central mass of small oil drops.
Asci: 8-spored, apex not blued in Melzer's Iodine, upto 250µm long, but mostly smaller.
Paraphyses: 2µm diam, slightly wider at the tips, curved at the tips, mostly unbranched, with granular orange contents which turn green in Melzer's Iodine"

Would you say Pulvinula miltina in my case?

Thank you again,

Ethan

I don't think so. P. miltina is an Australian species and maybe an endemic species.

Is there any modern key on Pulvinula, at least for Europe?

Best regards from Lothar

Hi Ethan,

I found yesterday what I believe to be the same fungus, here in Portugal. I found it on a stream margin over soil attached to debris and the mesurements are exactly like yours!
I found this document, and I think this could be P. miltina too. Since it has been found on different places over the world, it is possible that it is just rarely collected. 
The document - http://www.ascofrance.com/uploads/forum_file/Pulvinula-johannis-Sydowia-60-0247-0252-0001.pdf

Regards,
Elsa

All these collections (of P. miltina) were reported before the use of molecular analyses, so I'm quiet sure that a modern review should provide another diagnostic.
Last weeks, I studied five different collections of Pulvinula (in fresh state) and I was unenable to provide a correct name, based on the known data on this genus  :-(
Best.
Nicolas