问题:

I have uninstalled web from extension and installed it again since than I am getting this error while i try to open the admin. Front end is working fine.Please help

回答1:

It is very important to clear the Compiler cache and after that turn it on in backend of Magento.

Clear the cache:

SSH:
find ./var/cache -type f -delete
FTP: 
mrm -r ./var/cache ; mkdir ./var/cache

Disable/Clear Magento compilation

SSH:
php -f shell/compiler.php -- disable
php -f shell/compiler.php -- clear
FTP:
mv ./includes ./includes.unused

回答2:

Ok, so Debug. Go to app/Mage.php. Find register function add in

if ($key == '_singleton/VladimirPopov_WebForms_Model_Observer'){
    Mage::log(debug_backtrace(false), null, 'backtrace.log');
}

Do this right before if (isset(self::$_registry[$key])) {

Then go to app/var/log/some_filename.log. There should be two entries. One is right and one is wrong (read duplicate). That should help you to find the offending file/function.

NOTE: Be sure to backup app/Mage first and restore it after. This is a critical file.