Unknown encoder 'libfaac'
libfaac has been removed from FFmpeg for licensing reasons. So instead of using the following option to encode AAC:
Use FFmpeg's experimental encoder instead:
-acodec aac -strict experimental
In WinFF, you can go into Edit>Presets and make this change to the affected preset, then click "Add/Update" and then "Save" when you are done.
Of course, the experimental codec is just that. YMMV. If you really want libfaac, there are special unofficial builds of FFmpeg on the web that include libfaac.