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.
In this case, all projects were private, visible only to members. Anonymos users and Non members had no permissions to view issues. As the user was a member of none project he couldn`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.