Problem: When external users want to join a project and try to do so using the link in the welcome email they received, they are not requested to enter a password. Rather, they are confronted by a login mask with no known access data.
Possibly, the one-time token in the welcome email is no longer valid.
Some virus scanners automatically open links in emails. Thus, the one-time token becomes invalid. Please request the external user to (temporarily) disable the virus scanner.