Is there a free tool that allows for cutting of videos? I am downloading AVI files from a German "online" video recording service. These files don't work with quicktime, but using the Elmedia video player (via app store) does the job. 

Now I would like to "cut" those videos (basically selecting the real content of the videos, as they always contain additional 5 minutes before/after the actual recording).

I installed avidemux (using *brew cast install avidemux), and the tool installs. But when I open these AVI video files ... it seems that the buttons to select A/B boundaries for cutting are simply disabled. And on top of that, audio isn't in sync when avidemux is playing my AVIs - so I doubt that avidemux cutting the video would give me a correct file in the end.

As mentioned: quicktime refuses to play my avis,so this existing question on how to trim using quicktime doesn't help me.

Thus: are there other tools for Mac Os that work better?
You could just put them into iMovie and trim the beginning and end and export them.

I tried using iMovie with .avi files before posting this answer, so I know it works.