edit: actually this might have been a 'recent' recipe rather than a 'favorite' recipe, I suspect this bug applies to both.

the recipe in question belonged to a mod that was removed from the save, expanding the FNEI interface (down arrow) triggers this crash.

crash log:
Error while running event FNEI::on_gui_click (ID 1)
Unknown sprite "recipe/sulfuel-mk4"
stack traceback:
FNEI/scripts/gui.lua:186: in function '?'
FNEI/scripts/gui.lua:148: in function 'add_gui_template'
FNEI/scripts/gui.lua:157: in function 'add_gui_template'
FNEI/scripts/gui.lua:157: in function 'add_gui_template'
FNEI/scripts/gui.lua:157: in function 'add_gui_template'
FNEI/scripts/hotbar/gui.lua:138: in function 'draw_hotbar_bar_extension'
FNEI/scripts/hotbar/controller.lua:41: in function 'open'
FNEI/scripts/hotbar/controller.lua:156: in function 'cur_event'
FNEI/scripts/custom_events.lua:64: in function 'invoke'
FNEI/scripts/events.lua:75: in function <FNEI/scripts/events.lua:72>

workaround: disable FNEI, load save, save the game, re-enable FNEI, load save

Thanks for your feedback.
Now I do not have much time, but I'll try to fix it.

fixed in 0.1.9

How can recipes be removed from the favorite ones?

alt + click on recipe in hotbar