My video thumbnail displays as a grey YouTube error thumbnail
This help article relates to Performance YouTube Player for WordPress - a plugin that is for sale through CodeCanyon.
When you see a grey YouTube error thumbnail the most common reason for this is that the video embed is trying to display a webP format image and there is not one available. The YouTube video doesn't have a webP thumbnail.
YouTube does not always create webP images, especially for older videos, and many plugin developers do not allow for this.
Performance YouTube Player for WordPress fixes this and gives you the option to disable WebP in your embedded video and display the default JPG thumbnail.
Once you have inserted the Performance YouTube Player block, go to the options and choose "Disable WebP" in the Blocks settings to display the JPG version.
Another solution to fix a grey thumbnail from YouTube is to edit the video on YouTube. In Edit mode, change the thumbnail to another, then save. Edit the video again, then re-select the thumbnail you want. This should trigger YouTube to regenerate the thumbnails, including a modern webP version.
Not all YouTube videos have webP images for thumbnails, so older videos need to have their thumbnails regenerated to generate a webP thumbnail.