Thanks for reporting.
MP4 opus audio stream failure - Caused by the attempt to mux the downloaded audio codec opus into an MP4 container. MP4 does not support opus so extra conversion is required.
WEBM VP9 video stream failure - VP9 is experimental in version of ffmpeg we distribute and is failing to mux.
Will handle this in the next release.
In the mean time select an MP4 H264/AAC stream and this will download and mux correctly. This is most commonly what appears first in the list of available streams from youtube.
Occasionally Jaksta fails to download specific YouTube videos. It works most of the time, but certain videos fail everytime I try (for example: https://www.youtube.com/watch?v=phUhvp812Mo). Jaksta appears to proceed (I can see the progress as if it is normal) through the lengthy download process but ends with a error and the file ends with zero length. I don't have this problem when using "YouTube by Click" free software.
Is there anything that I can do to help Jaksta succeed?