I could recover some..
.. but not all. The OPML-file was cut in half (probably at the position, where the disk got full). I'll try to re-add the missing feeds later this day. And for now I stopped the donkey to keep my home-partition reasonably free. At least aKregator repaired the OPML file, so that it now has linebreaks again and dunno what else - vi loads it quickly now again.