While connecting the Add-In to the Redmine account the error message appears:
It something wrong with your Redmine service. Please check the version of Redmine. This add-in compatible with Redmine 2.5+
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.
Case for example: all projects are private, visible only to members. Anonymos users and Non members have no permissions to view issues. As the user is not a member of any project he can`t connect the space either.
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.