Thanks for your answer and sorry I'v been so slow in replying. I had forgotten my password and since I replaced nRelate with YARRP on the website that I was working on the problem was sort of solved.
However, I use nRelate on serveral websites and the problem wasn't only on that site, it exist on all sites where I use nRelate. I've tried ignoring the problem hoping that new releases would fix the problem but yesterday I updated to the newest version and pressed the re-index-button. Bad idea.
Before re-indexing nRelate displayed excerp on old post and it was only new posts that didn't get one. But by pressing re-index all excerps on all posts disappeared.
For example, in the the right sidebar at
I have nRelate popular content. The first on the list (at the moment) is a page (and has an excerp), the rest (posts) used to have excepts until I re-indexed the site yesterday.
The list at the bottoms should be related content with excerps. All old posts (but no new) had excerps until I re-index the site yesterday.