When things go wrong your video is basically unplayable or the first video plays fine and then freezes when moving on to the next. Generally if both videos weren't produced with the exact 100% same settings you will have issues. You can try the basic concat but it often won't work right.
Solution for me:
My example uses 3 videos in total so "n=3" and a=1 to include audio.
ffmpeg -threads 12 -i file1.mp4 -........
#if you have nvidia make sure you install the nvidia-cuda-toolkit so hardware acceleration can be used
tar -jxvf ffmpeg-3.3.2.tar.bz2
install prefix /usr/local
source path ........
Jun 1 15:45:42 videoeditor-desktop org.mate.panel.applet.MintMenuAppletFactory: project_data:INFO Missing folder chosen by user:
Jun 1 15:45:42 videoeditor-desktop org.mate.panel.applet.MintMenuAppletFactory: project_data:INFO Removed missing file: MAH02949.MP4
Jun 1 15:45:57 videoeditor-desktop org.mate.panel.applet.MintMenuAppletFactory: ui_util:WARNING Icon theme media-playback-start not found. Will use backup........
Movie-Aspect is undefined - no prescaling applied.
videocodec: libavcodec (720x1280 fourcc=34504d46 [FMP4])
[mpeg4 @ 0x2c91c00] timebase not supported by mpeg 4 standard
Could not open codec.
FATAL: Cannot initialize video driver.
That is the error I would get, I thought it was a codec error something else. By fluke Isaw something about ffmpeg where someone recommended manually telling it to the "fps" and it worked.........