To overcome this problem with joomla backend ,
1. You can request support to allow save_session_path
OR
2. Create a new folder with chmod (777) and put it above in /public_html/ folder and add to your .htaccess file this line:
Code:
php_value session.save_path '/home/cpanelusername/public_html/newwriteablefolder'
Note : Make sure the path to Joomla should be the same as your mosConfig_absolute_path with the additional folder added with no trailing backslash added .
3. If the account is hosted on phpSuexec server then you can create a php.ini file under public_html and set
Code:
session.save_path = /tmp