I had
exactly the same error message when upgrading from 3.50 to 3.60 so I can tell you that most probably you didn't upgrade nopCommerce properly.
The process is as follows taken from (here
http://docs.nopcommerce.com/display/nc/Upgrading+nopCommerce) - in brackets are my comments (as the upgrade instructions seemed ambiguous and vague to me at first, too):
1. Get the latest nopCommerce package.
(Actually you have to download two packages - first - the non-source upgrade package and second - the sql upgrade package in a zip file).
2. Make a backup of everything in your site, including the database. This is extremely important so that you can roll back to a running site no matter what happens during migration.
3. You have to upgrade stepwise. For example, if your current version is 2.00 and the latest available version is 2.30, then you have to upgrade to 2.10, then to 2.20, and then to 2.30. So download the required upgrade scripts from our Downloads page.
4. Once an upgrade script is downloaded, execute it over your database
(meaning the sql script from the second zip file - in Microsoft SQL Server Management Studio).5. Remove all
(meaning ALL) files from the previous version. As you deploy, make sure that the target App_Data\Settings.txt and App_Data\InstalledPlugins.txt files aren't overwritten, so that the production site continues to point to the production database.
6. Upload the new site files
(meaning the non-source upgrade files from the first zip file )7. Copy back Settings.txt and InstalledPlugins.txt files into App_Data directory
(from your backup)8. Ensure that everything is OK
(clear webbrowser's cache, or load directly from the webserver - F5, restart apppool and website if necessary to see the changes).