Thanks for your answer, @OttoKerner. I disabled the checkpoint but I still can’t have the video to play. It doesn’t show any error but, after about a second where it starts to play the video (same it did with the checkbox on, before showing the error message) and then it stops to a black screen.
All this happens in a Photos library, movies and tv series ones doesn’t show any problem playing videos. But this one is a .MOV, and I’ve no idea if it’s a specific problem of Plex with this format.
Edit: it doesn’t really stops, it’s just very slow, like 10 seconds and then starts to play it. I guess there’s some problem with accessing the file (it’s about 150 MB), or maybe it needs to load it all in memory before to play? I’m going to do some more test on this.
Ok, it seems a problem of Plex playing .MOV files. I converted it to .MP4 and, despite the fact its size is bigger, it plays it smoothy. I have to remember not to use .MOV files in the library. 
I can’t use Plex for Mac because it gives me this error on photos:
It seems I can only get a smooth experience accessing the server via IP, and the app seems not to do that.
