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Your ffmpeg was not compiled to include support for any H.264 encoders, so it is forced use the encoder named mpeg4 which outputs the old format MPEG-4 Part 2 video. This is a legacy format which was common before H.264.

Your device/player/app/browser probably doesn't support MPEG-4 Part 2 video.

Output H.264 instead.

• Download an already compiled ffmpeg that has what you need. Put it in your PATH (see FAQ), or provide the full path to the new ffmpeg in your script.

• Or re-compile ffmpeg with --enable-gpl --enable-libx264. See FFmpeg Wiki: Compile & Install FFmpeg on CentOS/RHEL/Red Hat.

Tell it which audio you want with -map:

The issue was the order of the args. If I order them this way:

After spending some more time, I decided to start from something easier for understanding (easier logic). I just remove -ss/-to and modify the -filter_complex by spliting the original video into 3 streams, from which I cut out the corresponding parts. The last 2 streams of the same length I have superimposed on each other using the method from my question. After that, I re-combined them in the correct sequence.

(For clarity, I enclosed variables in more and less signs.)

You can't re-use labels (a.k.a. "pads", unintuitively) from filter outputs. So you can't use si more than once.

Add the split filter to make copies:

Some videos do not work because Youtube does not always expose their video's urls when requested.

Youtube-DL makes a request to http://www.youtube.com/get_video_info?video_id=XXXX (where XXXX is the video ID) and this returns a JSON text file which has a listing of file urls for the MP4 / webM / M4A, OGG etc, associated with the requested video upload.

For example:

(1) Is working: You can find googlevideo.com links (MP4 etc) mentioned inside the JSON
Pinocchio (trailer) : http://www.youtube.com/get_video_info?video_id=y8UDuUVwUzg

(2) Not working: You cannot find googlevideo.com links in this JSON (no video links to download)
Mirage (full film) : http://www.youtube.com/get_video_info?video_id=FTY-L-l3KN8

What if you have an input for each of the possible radio button options, but only show the one that's selected? You can set up pattern-matching callbacks for all the inputs, and use just one at a time. Perhaps a change to the radio button selection would not only change which options are shown/hidden, but clear all of them as well, so you can be sure to accept only entries in the correct one.

Edit: This really goes beyond the scope of a single question, so I'm leaving out a lot of details here to keep from writing an entire Dash app. I hope this helps. These are some possible function definitions for each of the different types of callbacks I mentioned in the comments.

Last [outv] shouldn't have semicolon.

So instead of

FILTER_FINAL_LINE="${FILTER_FINAL_LINE} concat=unsafe=1:n=22:v=1:a=0[conc]; [conc][logo] overlay=50:50 [outv];"

FILTER_FINAL_LINE="${FILTER_FINAL_LINE} concat=unsafe=1:n=22:v=1:a=0[conc]; [conc][logo] overlay=50:50 [outv]"

is correct

https://superuser.com/questions/1395235/no-such-filter-in-ffmpeg

The trim/atrim filter has to decode the video until it reaches the desired timestamp. This can be slow.

The reason I suggested trim in your previous answer is because you initially wanted to use frames (instead of timestamp) as the unit which trim can do, but -ss + -t/-to can not.

You can use -ss + -to/-t input options to quickly jump to the closest keyframe then start decoding.