If you are sure your config files are correct (be sure) you can either use phpmyadmin to fix the cache key or use this trick to bring up admin and then fix it in the sessions tab.

In includes/functions/sessions.php

find: (about line 154)

Code:
  function zen_session_save_path($path = '') {
    if (!empty($path)) {
      //return session_save_path($path);  // comment this out
	  return "/proper/cache/path";  // add this
    } else {
      //return session_save_path();  // comment this out
	  return "proper/cache/path"; // add this
    }
  }
note commenting out the return lines and hardcoding the proper value temporarily.

I've seen the fix_cache_key come up with a blank page. Don't know why.

Be sure to set the proper path in the admin and then undo the above temp fix.