This video says about itself:
Scarce Tortoiseshell Feeds on Oak Sap ヒオドシチョウがミズナラ樹液を吸汁
9 February 2014
A Scarce Tortoiseshell (aka Yellow-legged Tortoiseshell; Nymphalis xanthomelas japonica, family Nymphalidae) feeding on the fermenting sap of an oak tree (Quercus crispula, family Fagaceae). October 2013 in Japan.
Translated from the Dutch Vlinderstichting entomologists:
July 14, 2014
This week a butterfly species entered our country which had never been seen before in the Netherlands: the scarce tortoiseshell. This involves dozens of individuals. This species was initially unnoticed because it is very similar to another one: the large tortoiseshell. An expert from the Butterfly Foundation discovered that many sightings of large tortoiseshells reported since late last week were incorrect. It was in all cases the scarce tortoiseshell.
Usually, this species, new for the Netherlands, lives much further to the east.