You were trying to renew your site license, set up a Citavi account, reset your password, etc., but you never received a confirmation email.
When the email finally does arrive much later than expected and you click the link, you receive the message that the confiramtion link is no longer valid or that the email address was already confirmed.
Reason
Your institution or your email provider uses the method of greylisting to defend against unsolicited emails. The mail server of your institution or provider rejects the acceptance of mails from unknown deliverers at the first delivery attempt.
The emails from our server are rejected first, but then our server tries re-sending the messages again.
Solution
Please wait until the message reaches you and do not request additional confirmation emails in the meantime.
If you have already requested additional emails, please wait until they reach you. For security reasons the links from earlier emails are disabled and only the link in the last requested email will work.
Due to the delay, it can happen that the emails are not delivered in the correct order by your provider, so you may have to check the previous links as well.