Errors after upgrading PHP to 7.4 WordPress

That sounds like a theme problem, since you have all plugins disabled. The theme has some error, possibly some extra spaces at the top of a file.

I’d change to one of the ‘Twenty’ themes and see if the problem stops. If so, contact your theme’s support area.

If problem persists with one of the “Twenty” themes active, then I’d try a re-install/re-update of the WP core code, using the Admin, Updates screen.

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