The problem is comes in checkout.xml file when you are upgrading to Magento 1.4.1 to Magento 1.5.1.This file located in app/design/frontend/base/defualt/layout/”yourththemehere”/checkout.xml. By removing this error you have a multiple option that is replace the checkout.xle file in the default theme foleder and than overwrite it in your theme.Make a point remove the Magento cache memory .The changes is done after flush the Magento Cache.
I just had the exact same problem. After spending two hours, I just changed permissions of the parent directories of my magento installation to 777 and now filesystem is active.
If you wish to use web based downloader for installation, upgrades or MagentoConnect extensions installation, you will need to have write permissions on ALL magento files including the root Magento folder.
Files and folders will need to be returned to 655 non-writable permissions after installation.[...]
Don’t forget to do
find . -type d -exec chmod 775 {} ; find . -type f -exec chmod 664 {} ;