Classic USB. Plastic. (I don't know the exact model, I got it from work.)
After adding a site or a computer it works a few days and then suddenly when I try to use it with my phone or computer I just get an error about no <something>.
Not enough info to even guess what might be wrong, but I'd assume it's defective and I'd try with another key as well. I wish you good luck with the next Yubkey you get! :)
Btw. this is the first time I've read on a public forum that someones Yubikey is defective, they are really well made and I didn't manage to break one via regular use and bad maintenance.
After adding a site or a computer it works a few days and then suddenly when I try to use it with my phone or computer I just get an error about no <something>.
So yes, probably defective.