In the last week I generated a SSH id_rsa key on my localhost that I have copied in authorized_key file at the right place on the remote server I want to reach without entering password.
This worked fine.
Now this morning trying to log again without password, it seems that the key is now invalid, I am asked for a password.
I have retried to generate a new key, and copy it again on the remote server authorized_key and it now re-works !
My question: Why was the initial key no more valid ?
I don't know any way to expire a key (you need a certificate), so I think somebody (ISP) could be override the key for maintenance tasks, or you have override your private key in your local machine. I think the best response is to be sure that your local public key and the remote public key are the same.