I had two occurrences up till now when the program begins repairing the rar set, but stops in the middle of it and leaves .rar and .rarx files in the download folder. Because .rar files are then incomplete when I try to quickpar repair again it is not working, but if I append the .rarx files to .rar files I get valid archives that are already repaired and I can unrar the set manually.
I am not sure what causes this behaviour, just wanted to report it.