Safari will not open - error notice 303
What can I do to open Safari when I have error message 303
MacBook Pro 15″, macOS 10.15
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Clear all website data
https://support.apple.com/guide/safari/manage-cookies-and-website-data-sfri11471/mac
Enable Develop menu, click Develop in the menu bar and then Empty Caches from the dropdown twice.
https://support.apple.com/guide/safari/use-the-developer-tools-in-the-develop-menu-sfri20948/mac
Related: If Safari doesn't open a webpage or isn’t working as expected