Prerequisite: The project type is "personalized survey"
One reason for this can be: The session duration has expired, e.g. because the participant has paused the survey.
Solution: The participant simply has to click on the invitation link again.
Another reason for this can be: The distribution link is incomplete (because of sloppy use of copy&paste e.g.
Solution: The participant has to check if the link is complete
Under Survey Settings > Login Options > activate the checkbox for "Allow parallel access".
This setting prevents mistakes when using e-mail URL scanners, such as those used in Office 365.