I recall some strangeness with recently re-added OGG/theora output. I found this comment in old CinCV repo: https://repo.or.cz/cinelerra_cv/ct.git/commit/5d7c9bfd9dbfa3c4ae673a8c8b3d35... "r636: j^'s ([email protected]) patch to mux ogg in a way compilant with specification y - makes oggz-validate happy and probably some players also" I followed app name and landed here: https://www.xiph.org/oggz/ Oggz Oggz comprises liboggz and the tool oggz, which provides commands to inspect, edit and validate Ogg files. The oggz-chop tool can also be used to serve time ranges of Ogg media over HTTP by any web server that supports CGI. liboggz is a C library for reading and writing Ogg files and streams. It offers various improvements over the reference libogg, including support for seeking, validation and timestamp interpretation. Download The latest version is 1.1.1, released Apr 29 2010. http://git.xiph.org/?p=liboggz.git;a=shortlog git has some improvements over it .... I hope it will helpful in catching those strange errors about not marked keyframes ...