Ok, thank you :). After removing the expiration date the healtcheck does not report any problems anymore.
However, I am still unable to do an account recovery. After providing my private key it immediately says
This key doesn’t match any account.
But I am 100% sure that the private key I provided is in fact the private key of my account. I have used it recently (before 2019-01-30) to access my account from a different machine.
There is another thread about such a problem: Cannot recover user due to 'Key doesn't match any account' after server move
However, I don’t really understand the technical background in this case. Other than the key expiring, nothing changed on the server.