Brother MFC Scanner not recognized any more
Solution 1:
I experienced the same problem with macOS High Sierra 10.13.6 on a MacBook Pro 13" 2018 and a Brother MFC-j6510DW.
The solution is straightforward, you have to reset the printing system according to the Apple support document, macOS Sierra: Reset the printing system
Go to System Preferences → Printers & Scanners:
Control click in the list at the left, then choose Reset printing system... from the context menu that appears.
After you reset the printing system, the list of printers in Printers & Scanners preferences is empty.
Re-add any printers
That's all.