Magento – Mage registry key “_singleton/weee/observer” already exists

Ok so I am not sure if this is the bug but if you compare the Magento 1.8 and Magento 1.7 versions of the function currency in Mage_Core_Model_Local you will notice a reference to a bug with Zend_Currency

Magento 1.7: Mage_Core_Model_Local->currency

/**
 * Create Zend_Currency object for current locale
 *
 * @param   string $currency
 * @return  Zend_Currency
 */
public function currency($currency)
{
    Varien_Profiler::start('locale/currency');
    if (!isset(self::$_currencyCache[$this->getLocaleCode()][$currency])) {
        try {
            $currencyObject = new Zend_Currency($currency, $this->getLocale());
        } catch (Exception $e) {
            $currencyObject = new Zend_Currency($this->getCurrency(), $this->getLocale());
            $options = array(
                    'name'      => $currency,
                    'currency'  => $currency,
                    'symbol'    => $currency
            );
            $currencyObject->setFormat($options);
        }

        self::$_currencyCache[$this->getLocaleCode()][$currency] = $currencyObject;
    }
    Varien_Profiler::stop('locale/currency');
    return self::$_currencyCache[$this->getLocaleCode()][$currency];
}

Magento 1.8: Mage_Core_Model_Local->currency

/**
 * Create Zend_Currency object for current locale
 *
 * @param   string $currency
 * @return  Zend_Currency
 */
public function currency($currency)
{
    Varien_Profiler::start('locale/currency');
    if (!isset(self::$_currencyCache[$this->getLocaleCode()][$currency])) {
        $options = array();
        try {
            $currencyObject = new Zend_Currency($currency, $this->getLocale());
        } catch (Exception $e) {
            /**
             * catch specific exceptions like "Currency 'USD' not found"
             * - back end falls with specific locals as Malaysia and etc.
             *
             * as we can see from Zend framework ticket
             * http://framework.zend.com/issues/browse/ZF-10038
             * zend team is not going to change it behaviour in the near time
             */
            $currencyObject = new Zend_Currency($currency);
            $options['name'] = $currency;
            $options['currency'] = $currency;
            $options['symbol'] = $currency;
        }

        $options = new Varien_Object($options);
        Mage::dispatchEvent('currency_display_options_forming', array(
            'currency_options' => $options,
            'base_code' => $currency
        ));

        $currencyObject->setFormat($options->toArray());
        self::$_currencyCache[$this->getLocaleCode()][$currency] = $currencyObject;
    }
    Varien_Profiler::stop('locale/currency');
    return self::$_currencyCache[$this->getLocaleCode()][$currency];
}

My suggestion would be to try updating the try catch to match that of Magento 1.8 (I know this is not the best solution and obviously I would not suggest editing the core files but adding a rewrite)

Your PHP developer should be able to solve the exact problem with this trace, but to explain the Problem:

You have an observer which calls rkt_jscssforsb/observer as singleton (the default) but this model could not be loaded, so it saves false in the registry, which is not recognized by the next call of this singleton as it has the value false, so it is tried again to call this singleton, but the registry does not allow to write a value, which already exist.

Thats why you get the error message:
Mage registry key "_singleton/rkt_jscssforsb/observer" already exists

As you noted in your comment, you uninstalled a module, if this module had the name rkt_jscssforsb you should first try to clean the cache, maybe forcefully by delete magento/var/cache/* or try to grep/search trough the code for the occurence of it to clean up the rest