Important Information

• mpv 0.32.0
• macOS 10.14.5
• Homebrew installation

Reproduction steps

1. Set screenshot-format=webp in mpv.conf
2. Try to take a screenshot
3. "Error writing screenshot!"

Expected behaviour

mpv should save a webp screenshot (its support is confirmed in #6832 and referenced in the manual).

Actual behaviour

mpv fails to open libavcodec encoder (as can be seen in the log), leaving just a zero bytes webp file.

Log file

output.txt

Latest IINA based on mpv 0.32.0 behaves the same.
Taking a webp screenshot with FFmpeg in terminal works, so it's most likely an issue with mpv.