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This happens because of the privacy restrictions of the organization inside Redmine space. Check that user is a member of an active project in the space and has permission to see the issues.
In this case, Case for example: all projects were are private, visible only to members. Anonymos users and Non members had have no permissions to view issues. As the user was is not a member of none any project he couldn`t can`t connect the space either.
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