Session Expired problem after installed the stock per atribute plugin

Details: Session Expired when login in, registering, adding item or checking out.
Version: 1.5.1
Hosted: godaddy.com
Instalation: Automated by godaddy
Template: Got one from themeforest
Plugins: PagSeguro, Quantity per Attribute (Stock), Correios (SEDEX adapted)
Site: http://www.usastylebr.com

Start happening: After installed the stock per atribute plugin.
Details:
The site was fully installed, with template and other plug-ins working fine, i installed this last one and stopped working, i tried to uninstall but still doesn't work and the problem is, i already did a lot os customizations, translated all pages and can't just restore all back to original, can someone help me debugging it or something like that? I can grant access to the server and ftp or send the files. Actually, can someone help me at all? Any help will be much appreciated.