New Install Civi 5.4.0 getting Uncaught Symfony Rename Error and Blank Screen
Problem resolved.  The bug is in fact that the PHP rename function running on Windows Server can't handle a double slash (\\) in the path name.  I made three changes (so far).

Symfony - Filesystem.php line 272
        if (true !== @rename(str_replace(array("\\\\", "\\/"), "\\", $origin), str_replace(array("\\\\", "\\/"), "\\", $target))) {  

Smarty.class.php line 1253
       $_smarty_compile_path = str_replace(array("\\\\", "\\/"), "\\", $this-&gt;_get_compile_path($resource_name));  

Smarty - core.write_file.php line 45
        @rename(str_replace(array("\\\\", "\\/"), "\\", $_tmp_file), str_replace(array("\\\\", "\\/"), "\\", $params['filename']));