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 aa-in compatible with Redmine 2.5+
This happens because of the restrictions of the organization inside Redmine space. Check that user has access (is a member) to the projects in the space and have permission to see the issue.
In this case, all projects were private, visible only to members. As the user was a member of none project he can`t connect the space either.