From my extensive usage with it, it has like a 90% success rate. Based on my experience, this error shows up because:
1. The video you're trying to download has been recently uploaded. Try again in a couple days or a week.
2. The video requires a direct purchase. Some videos, despite being under the Red tab, do not play with a Xvideos Red subscription. However, on the rare occasion, a video for direct purchase does get processed, but only plays in a very low resolution.
3. The video requires a channel subscription. Same reason as the previous point, but I've never had these processed.
4. Unsure of this one, but perhaps it gets uploaded to a different server? Some videos that are neither for direct purchase nor channel subscription and that should work with a Xvideos Red subscription, just do not work. Seemingly random.