Can you please add a new section called "Digital restriction management (DRM) incompatible" ("restriction" is recommended for DRM in https://www.gnu.org/philosophy/words-to-avoid.en.html) or similar in https://notabug.org/GPast/avideo/src/master/README.md and describe that DRM restricted videos cannot be downloaded with AVideo? Let us know which container file formats that are affected.
Text from #5 that can be used:
In order to circumvent YouTube's DRM and gain access to the media file's URL, either some JS downloaded from YouTube needs to be executed (which avideo is built to avoid), that JS needs to be reverse-engineered (not currently practical), or YouTube needs to remove its DRM option (which doesn't seem likely in the short term). As a consequence, it's currently impossible to download those videos with DRM using avideo.
I'm extremely sorry about this, and hope it becomes possible to resolve it. If you need access to DRMed videos and are OK with the proprietary JS, YouTube-DL is able to do this.