MSM PHP errors can't edit General Configuration

[eluser]Unknown[/eluser]
I'm trying to solve a problem of this error :

Your system folder path does not appear to be set correctly. Please open the following file and correct this: index.php

When I'm in MSM on my second domain, I can't edit the General Config info. I get these erros upon save:

Code:

Undefined index: base_path

A PHP Error was encountered

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at /home/partners/public_html/cms/codeigniter/system/core/Exceptions.php:170)

Filename: libraries/Functions.php

Line Number: 809

A PHP Error was encountered

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at /home/partners/public_html/cms/codeigniter/system/core/Exceptions.php:170)

Filename: libraries/Functions.php

Line Number: 414

[eluser]jairoh_[/eluser]
in your index.php specify the path of your system folder

Code:

$system_path = 'system'; //locate the path

[eluser]Unknown[/eluser]
This did not work. Still getting the error. Any ideas?