Official comment
@Martin > Sorry to hear about that, but unfortunately for both of us, those error messages don't tell me much about that file in particular.
I can tell you the path+filename isn't over 255 characters, so that's not it. It doesn't appear to have any odd characters (such as ? or * in the file name), and no accented characters either, so that leaves the file format as the main culprit.
Just because a file is "MP4" doesn't mean it can / will play within rekordbox. Keep in mind that MP4 is just a wrapper for audio and video codecs. Here are majority of the video codecs supported within MP4:
HEVC/MPEG-H Part 2/H.265,MPEG-2 Part 2, MPEG-4 ASP, H.264/MPEG-4 AVC, H.263, VC-1, Dirac
And here are the audio codecs:
MPEG-2/4 (HE)-AAC, MPEG-1/2 Layers I, II, III (MP3), AC-3, Apple Lossless, ALS, SLS, Vorbis
So it's entirely possible that you don't have the proper codec installed in your computer, the codec used by that file is not supported by rekordbox, or is in a bit / sample rate outside of the supported range.