Can't add member to the project

Hi, I am just experiencing an issue when trying to add a member to the team. I invited the member via email, but when he tries to sign up, he gets a message "Sorry, the confirmation has been expired". The same for other email addresses. We tried different browsers, clear the cache but none of these worked. It is kind of weird as the invitation worked for other members. Do you have any idea to resolve this issue.

Thanks, Jan