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This happens because of the privacy restrictions of the organization inside Redmine space. Check that user has access ( is a member ) to the projects of an active project in the space and have has permission to see the issueissues. In this case,
Case for example: all projects were are private, visible only to members.
Anonymos users and Non members 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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Solution:
Connect to the administrator of your Redmine space.
The administrator should include you in an active project or change permissions for Non members in the Administration panel → Roles and Permissions → Non member.
Permissions with view options should be checked.