If the scan-to function of your networked Brother MFC mysteriously stops working, it may be because of strict firewall settings. The scanner attempts to connect to the Brother ControlCenter software on your PC via a directed UDP connection to your computer. If this is blocked at your computer (by e.g. a firewall), the scan function no longer works.
To resolve, open up port UDP 54925 in your firewall.
For Network PC-FAX, you also have to open port UDP 54926.