Thanks for the respose. I think it was some kind of one time error. I decrypted all files again, wrote them onto a new DVD+R and it seems to work fine now.
As far as files went, I did write only selected files: original DVD has 4x3 and 16x9. So I got rid of 4x3 files since I don't need them. If I select 4x3 from the menu, it gets confused obviously, but 16x9 plays fine now. And anyway, it was some kind of encoding problem in the middle of the chapter.
Anyway, all is well now. Thank you.