Ajax Error when trying to view relationships tab
I had the same issue.  CiviCRM 4.7/Drupal 7/IIS8 configuration.  The Ajax call was generating a query longer than what the web server was configured to handle and behind the scenes the web server was throwing an error, but it is not logged to the Civicrm log nor the Drupal log.

I was able to solve this by increasing the query string length in IIS.  


In IIS Manager select the site
Choose the Request filtering icon
Navigate to the query string tab 
Click the edit feature settings
Change the Maximum query string (Bytes) from 2048 to 4096 or something larger


Contact Relationships now show properly.