I had exactly the same experience, Tattie. I followed the links as they provided them, made the challenge and they removed the warning.

There are NO ads run with any of my videos. I have not allowed, though I've been invited to. I find them annoying so I don't want them on my videos.

There are quite a lot of videos posted on Youtube that DO use other people's recordings and copyright protected compositions. THAT is what Youtube has to battle and be sure is licensed properly. Because the volume of new material going up every day is so enormous, users like us have to prove our case, sometimes. And the process that Youtube has established to do that (challenge their warning or challenge their prohibitions) seem to work. At least it's worked for me.