Lars, I cannot help much with culture advice of Eriopsis in Europe. However, I can give some information about the place where Eriopsis biloba grows in Venezuela.
Warm during the day, and quite cool during the night.
The plant grown terrestrial in silica sand
Eriopsis biloba on Flickr - Photo Sharing! or lithophytic on sandstone
Eriopsis biloba on Flickr - Photo Sharing!
no matter what the culture sheet say, this species grows in full sun! open wet savahnnas...
very often, the roots are under water for quite long periods (almost constantly for a up to 6 months during the rainy season!)
Maybe this can help to understand what is going on with your plant (not enough sun?)