LimeSurvey version: 6.15.4
LimeSurvey Cloud
Survey theme/template: fruity
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When creating a new survey in LimeSurvey and setting a participant list with an access code for each participant, entering an access code is not required. Instead, without access codes, it is possible to proceed to questions after activating the survey in closed-access mode. Does anyone know of a similar problem and how to address this issue? 
Regarding another survey created (likely) in a previous version of LimeSurvey Cloud, entering the access codes is required to proceed to the questions. I might miss some relevant information to be conveyed. Thank you for your experiences.

When creating a new survey in LimeSurvey and setting a participant list with an access code for each participant, entering an access code is not required. Instead, without access codes, it is possible to proceed to questions after activating the survey in closed-access mode.

This should definitely not happen. Is the survey active?

So you are saying, in the same installation (LS6.15.4) you have two surveys that behave totally different?

The one created in LS 6.15.4 is allowing to access a survey in closed mode without a token, while the survey that was created in a previous version requires a token in closed mode (as it should)?

Can you provide a little example LSS that shows the issue, with all the settings you did?