Here a Lachnum I am not quite sure about. It is white, to 0,6 mm diameter and stipe up to 0,9 mm. The surface blushes slightly on damage. The hairs are cylindrical or slightly clavate, fairly short, 20-50x3-4 µm, no chrystals. Eight spored, no croziers. The spores have no droplets:

(6.7) 8.2 - 10.4 (11.4) × (2.0) 2.05 - 2.6 (2.8) µm
Q = (3.2) 3.6 - 4.5 (4.7) ; N = 37
Me = 9.2 × 2.3 µm ; Qe = 4.0

The paraphyses are 50-65 x 4.7-6, with one or two bigger and some smaller droplets, protruding some 15-25 µm.

With these features I end up in subkey 4 of Zotto's key and there step 3 with three lignicolous species and an "unclear collection from Filipendula". This one was most probably growing on Filipendula ulmaria on a wet and swampy lake shore.

I compared to an L. pudibundum i found some weeks ago (on Prunus padus) and the main difference seems to be that the hairs are not capitate (with the exception of one hair) and the paraphyses are marginally more protruding. Is a host switch to Filipendula possible for L. pudibundum?