Problem with CiviCase
This looks like a bug. It's coming from CRM_Case_Page_Tab::preProcess(), which calls CRM_Case_BAO_Case::getCases(), which then returns a list that's keyed sequentially, but then preProcess checks for a key that matches the case id, so there is no match.

$userCases = CRM_Case_BAO_Case::getCases(FALSE, array('type' =&gt; 'any'));
if (!array_key_exists($this-&gt;_id, $userCases)) {
  CRM_Core_Error::fatal(ts('You are not authorized to access this page.'));
}


This appears to have inadvertently been changed as part of CRM-21461. I'll file a bug report.