I had the same issue, my Mac's Notification Center App was crashing as soon as I would start my computer. To fix it, I quit the NC App in Activity Monitor, waited for it to reboot and cleared all of my email notifications, the NC stopped crashing. I supposed because I had so many emails in there that I did not clear it would just over-run the application with data. I have never had to do that before Big Sur. Hope this helps someone.
dustbuster42
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