Why are my survey participants asked for an access code?

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.