This happened to me when I tried to open the scanner from within the system preference pointers and scanner. I think it happens because the printer/scanner driver is old, and does not go well with Big Sur. Brother, apparently, stopped issuing new drivers with OS 10.15.
Go to the Apple applications store, and download the most recent version of "Brother iPrint and Scan". Open the scanner, or initiate a scan, from within this application. when I initiate from within the new brother software, rather than from within apple software, it works.
Socialist
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