Seems like so many wordpress users are facing error messages just after upgrading their wordpress to version 2.8, including myself. Couple of hours ago I kept receiving this error message “Error 500 – Internal server error  An internal server error has occured! Please try again later.” What I did?, first i called 1and1 my host. After they told me it’s a problem on my end. I tried google search and tried various solutions like editing the config.php file,  creating a php.ini file etc, But none of that actually worked for me.
After sometime I got a new error message everytime I try to access my website, which is “Fatal error: Class ‘WP_Widget’ not found in /homepages/xx/xxxx/htdocs/wp-includes/default-widgets.php on line 15“.
After trying and failing everything I know and everything I could find on the internet, I finally decided to delete the ‘wp-admin’ folder,  ‘wp-includes’ folder and all the files from the root excluding .htaccess and wp-config.php.
Uploaded them again to the root folder and it WORKED! finally after two hours.

So if you’re getting these error messages, try this.

First back up all your folders and files from the root.
Delete wp-admin folder.
Delete wp-includes folder
Delete all the php files from the root, excluding config.php and .htaccess file
Last, upload everything you deleted back to the root folder.
Done!

The website should work now, give it a try.