Nailed it. Now it works 
Found an older advise from our ffmpeg wizard Kevin, to get info about my rtsp stream using ffprobe. This was the output of my probe:
Metadata:
title : Media Presentation
Duration: N/A, start: 0.000000, bitrate: N/A
Stream #0:0: Video: mjpeg (Baseline), yuvj420p(pc, bt470bg/unknown/unknown), 800x600 [SAR 1:1 DAR 4:3], 29.92 tbr, 90k tbn
Stream #0:1: Audio: pcm_mulaw, 8000 Hz, mono, s16, 64 kb/s
I suddenly realized that I converted the audio to AAC, but for video I used the 'clone codec' option. As a result I stored mjpeg chunks into mp4 containers, because I thought that my cam did send an H.264 stream. Not appearantly...
After I instructed ffmpeg to convert the video to H.264 (using ... -c:v libx264...) everything seems to work fine in my dashboard. Will have a look tonight whether my old cam can send h.264, to avoid having to do this conversion in my Node-RED flow.
@kevinGodell: your mp4frag nodes are a real piece of art. Must have been an awfull lot of development work to get it running. If you ever have some ideas to troubleshoot root causes of issues (like the one I had) more easily, please share it!
