[FFmpeg-user] Problem with mp4 and concat filter

[Ingo Brückl]

I have a file 1.mp4 (Video: h264, Audio: mp3) that has a duration of 90 mins
and a file second.mp4 (same Video/Audio). Both files play fine.

I'm trying to cut a piece from second.mp4 by

  ffmpeg -ss 0:12:09 -i second.mp4 -codec copy -t 0:01:19 2.mp4

and then to concatenate both with

  ffmpeg -f concat -i <(printf "file '$PWD/%s'\n" ./{1,2}.mp4) -c copy new.mp4

The problem is that new.mp4 only plays until the end of former 1.mp4. The
following 2.mp4-part is either still picture or kinda slow motion. I think
it's because of wrong timestamps because ffmpeg tells about DTS being
non-monotonous or out of order.

So how can I concatenate both files? ffmpeg is v2.8.6.