When I try to mux ec3/eac3 - Dolby Digital Plus to a mp4 container staxrip says mp4box doesn't support it when Mp4box actually do.
Well, the report itself is too concise but a good find. I can confirm it.
Currently in 2.1.7.1, AC3 muxing by MP4Box has no problem, but E-AC3(DD+) is refused.
MP4Box muxes E-AC3 too, so this outdated warning should be removed.

I will be fixed in the next build.
I confirm this issue is fixed in 2.1.7.2 released today.
We can close this issue now. 馃槂