Steps to reproduce:
1. Make sure you use Opencast with ffmpeg 4.0+
2. Use a workflow that adds a trailer (use animate operation i.e.) or something similar
3. The workflow fails in the concat operation.
4. ffmpeg throws the error " This syntax is deprecated. Use '|' to separate the list items ('0:d=1' instead of '0::d=1')"
5. This is caused by the ffmpeg filter "aevalsrc=0::d=1[silent1]". The two :: are hard coded in Opencast's composer-ffmpeg.
6. In older ffmpeg versions channels were sepearated by ":" in this filter. Now they are sepearated by "|". But we only have one channel (I hope that is still right) and so we probably do not need the separator at all and one ":" should be enough.
Workaround (if any):