Cannot determine default context. CRM_Contact_Form_RelatedContact should implement getDefaultContext
When I look at it on the public demo it almost seems like the link itself is a mistake, since it doesn't look like a link to manage case, it's a link to some other function. If you go to the case dashboard or the civicrm dashboard (as opposed to My Contact Dashboard), the links there are correct.

Edit: some more info:

What seems to be happening is that normally CRM_Contact_BAO_Relationship::getContactRelationshipSelector (which is the thing that builds that table) will call CRM_Contact_Page_View_Relationship::links, which has the correct link for manage case. However for My Contact Dashboard it calls CRM_Contact_Page_View_UserDashBoard::links, which doesn't set anything for the CRM_Core_Action::NONE entry in the links array, and so something gets confused somewhere.

Untested but I think adding this code:

CRM_Core_Action::NONE =&gt; array(
      'name' =&gt; ts('Manage Case'),
      'url' =&gt; 'civicrm/contact/view/case',
      'qs' =&gt; 'action=view&amp;reset=1&amp;cid=%%clientid%%&amp;id=%%caseid%%',
      'title' =&gt; ts('Manage Case'),
    ),


into the CRM_Contact_Page_View_UserDashBoard::links function might fix it.