Thank you for your response. I have found the cause of the problem. Not only the server certificates were expired, but also (some of) the client certificates. But not all of them. So for some servers I managed to add the remotes again, and for some not. The "Error: The provided certificate is expired’ was not for the server certificate (which I just renewed for another ten years), but the client certificate of the client for which I tried to add a remote.
What needs to be done when you get the above error is remove the client certificate on the client machine which can be found in ~/snap/lxd/common/config/. Both client.crt and client.key must be removed (or renamed) and then when you call lxc remote add it will generate a new client certificate.
So if your lxd server certificate expires, also check the client certificates and renew them too when needed.