Site Cookie Error, Redirection and Blank Pages after Upgrade

There are newlines being generated before your <!DOCTYPE> declaration, which you can see by viewing the source code of your site from your browser:

enter image description here

As Rarst suggested, there is probably some incompatible code that is causing this output to be generated. The extra whitespace can cause the issues you’ve described.

There are many resources on this site and others about troubleshooting whitespace appearing before the doctype declaration.

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