I own a Brother MFCL2710DN and wasn't able to scan when the printer is connected via USB. The official brother packages for the printer and scanner were installed. FInally the this page helped me to find the root cause and get rid of the error.

First of all I checked with lsusb the vid and pid of my printer

bus 001 Device 003: ID 04f9:0433 Brother Industries, Ltd MFC-L2710DN series

Then I added a file brother.conf in /etc/sane.d with following contents (vid and pid of lsusb command):

usb 04f9 0433

and checked in dll.conf there is a line with following contents

brother4

That's it. Now the scanner works with xsane.