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Re:Aegis Authenticator – Secure 2FA App for Android by eitland on Thursday November 03, 2022 @09:12AM (#33450427)
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>.

So yes, probably defective.

Re:Aegis Authenticator – Secure 2FA App for Android by zmxz on Thursday November 03, 2022 @11:13AM (#33452281)
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.