Odd this was supposed to go into bug forum, maybe a mod can move it please.
That's strange. Those cakes have always required you to have very few things in your pack to use them, but I made a cake only last night and removed slices from it to put into the hamper that I occasionally take to our Europa bank meet.
This is a known issue related to the function that creates the piece of cake erroneously counting all spells in spellbooks as items inside of your pack. This makes it so that if you have 2 full spellbooks in your pack you cannot slice you off a piece of cake.
I shall remain cakeless then lol.
Apparently in my haste to respond, I forgot to mention that there is already a problem report in the system on the cake/spellbook issue.
No big deal, just one of those odd things that has you baffled when you are unable to have your cake and eat it too. :P