This bugged me for years. I never wanted to reinstall Opera from scratch, though it was never necessary. Nearly everything went perfect except for some strange empty mail- and news entries. But one thing i could never get rid off 'til now, the really annoying message:another user is running this copy of opera
after i clicked a link in whatever other application and Opera was already running. Googled this problem every 6 months, now i found (my) solution(thanks to following post from GregChi); in short he suggested:
So simple it is, so hard it was to figure out.