We provided response in the support ticket you have opened. I am pasting it below so it cam be useful for our community:
The error is being caused by one of the scripts that get executed during the upgrade to Sitefinity 8, in which the scripts fail to be executed properly. In order to fix this problem you will need to view the WebForm that is attached to the article in your Sitefinity project after being logged in as an administrator.
The web form can be placed anywhere inside the project folder (root, another folder, etc.), the important part is to navigate to the page once logged in. For example, if the form was placed on the root, you would navigate to http://mysite.com/FixUpgrade.aspx in order to execute the fix.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items