Thank you for that screen recording, @Sebastian Manger ! That did reveal the issue: the error is shown because the imported .mp4 file is over an hour long. AE does not currently support showing the waveforms for footage that are over 1 hour in length.
If you trim the length of the .mp4 layer to less than an hour, you should see the waveform load shortly after. I've filed a ticket internally for us to review the one-hour limit or improve the error so that you would know why the waveform won't load