This error relates to version 1.6.6.x and above.

Any of the following warnings/errors are related to the same known issue and can be resolved by applying the fix below.
Other symptoms include seeing 404 error pages after installation of version 1.6.6.x


Message: mkdir(): Invalid path

Filename: drivers/Session_files_driver.php

Line Number: 136

--

File:

/file/path/to/application/libraries/Admin_controller.php

Line: 91

Function: library

--

File:

/file/path/to/application/controllers/Index.php

Line: 5

Function: __construct

--

Warning: Uncaught Exception: Session: Configured save path '' is not a directory, doesn't exist or cannot be created. in

/file/path/to/system/libraries/Session/drivers/Session_files_driver.php:138

--

A PHP Error was encountered

Severity: Error

Message: session_start(): Failed to initialize storage module: user (path: )

Filename: Session/Session.php

Line Number: 143

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The following are extracts of error/warning messages that may appear at the beginning of an warning message

CI_Loader->_ci_load

CI_Loader->_ci_init_library

CI_Session->__construct

--

The following are extracts of warnings/errors that may appear within a warning message

('Session', 'CI_', NULL, 'session')

('Session', NULL, NULL)


The Fix


Using an FTP program navigate to and download:

/application/config/config.php


Using Notepad or a similar TEXT editor (not Word) Open /application/config/config.php
Around line 385, search for:
$config['sess_save_path'] = NULL;


Change 'NULL' to either '/tmp'


The line should now read:
$config['sess_save_path'] = /tmp;


Save this file and Re-upload the edited file back to
/application/config/config.php