I've noticed this a few times. Youcast will appear to download the video successfully. The video will be named and dated correctly. However, when it comes to playback, only the audio will be heard. The video itself will be black.
I've been able to replicate the issue. Here are two such videos which have caused the issue:
https://www.youtube.com/watch?v=-6-ADt4Ijxc
https://www.youtube.com/watch?v=-fiYgSXepdc
I am separately able to use a python-based method to download the video ( YoutubeDL ), which works successfully.
I've not experienced this same issue with any other video.