Teil 49 of Harry Potter and der Stein der Weisen is either missing or inaccessible. I tried clicking on the link to it at the end of Teil 48, and it took me to Teil 50. I tried searching for Teil 49, but couldn’t find it - though that may be my limitation.
I’ve already read the book in English, so I know what happened - but golly, I’d love to read that moment in German where Hagrid tells Harry …
Perhaps this is just an error in the link at the end of Teil 48. Can it be fixed?
It probably shouldn’t even be available as a shared book as it is copyrighted.
@ericb100 is right on the copyright issue. That should really not be publicly available, it puts all of LingQ at legal risk.
That said, let’s assume there’s no fault on your part - assuming e.g. that you own a legitimate German copy of the text, maybe you even imported it privately.
I believe I have encountered this or a similar bug in the past in my own private imports - most likely, also Harry Potter 1 but in Russian 
I was able to access the missing lessons by going into “edit course” mode and then clicking the lesson from there, I think that fixed it, although it’s been a while.
Certainly not his fault. I think that book has been out there for several years, although I thought it had a lot less chapters. (much more incomplete)
In my line of work, I am acutely aware of copyright and permission issues, and I didn’t download it. It was prominently featured on the Lingq site, under “Books.” At least one other Harry Potter book is there as well. I’ve assumed that Lingq had obtained the necessary permission licenses and put them there.