Flaw in the story? *spoilers!*
In the end it becomes clear that it is not Katja's diary but her friends diary, and that not she, but Ellen has cancer.
However, the first time the diary - and subsequent times - comes into the story, it talks about numerous affectionate things with Katja's boyfriend Frank. This raises the question that is Katja's boyfriend an ex of Ellen's, is he cheating on her - and if so why does she, seemingly, not react to this. I just found the ending too abrupt and raises even more questions. I do not mind an 'open' ending, or even an abrupt one, but this ending just seems sloppy to me. Maybe I'm coming off as being to critical, but I did enjoy the movie.