if you're using ffmpeg to concatenate videos and get "Non-monotonous DTS in output stream" for one specific video no matter what you do to it (and it also breaks the audio or something), go back and check the framerate and audio sample rate of that video. it needs to be the same as all the others. you can set the sample rate by re-encoding that video with (e.g) -ar 48000 and the frame rate with (e.g) -filter:v fps=25,
more important advice (though really a note to self):
don't try to solve technical problems late at night if they can wait until the morning