This is the output from mpv:
ffmpeg library versions: libavutil 56.22.100 (runtime 56.31.100) libavcodec 58.35.100 (runtime 58.54.100) libavformat 58.20.100 (runtime 58.29.100) libswscale 5.3.100 (runtime 5.5.100) libavfilter 7.40.101 (runtime 7.57.100) libswresample 3.3.100 (runtime 3.5.100) ffmpeg version: 4.2.1 mpv was compiled against a different version of FFmpeg/Libav than the shared library it is linked against. This is most likely a broken build and could result in misbehavior and crashes. mpv does not support this configuration and will not run - rebuild mpv instead.
I guess this is a bug. Where can I file a report?
In the meantime I would like to downgrade ffmpeg, hoping that mpv will work again. How can I do that?