The jubilee cake is supposed to stack with other exp effects, i used 2 crown store experience scrolls before i actually tested with another because i read the events details, the effects do not stack, i received the same amount killing enemies with just the cake active, as i did killing with both active.
Sadly I have to confirm.
I just checked on killing random same guars close to Vet Davon's Watch, they gave
~300xp without cake and any other XP buffs
~600 with Anniversary cake so 100% more
~600 still with Anniversary cake and Psijic Ambrosia which should give additional 50% XP
In active effects I see both XP effects in a same time, just without stacking effect
I spent most of my money to buy 35 pcs. Ambrosia 3k each for this event, when it seems I just lost my money .
The promise: This buff stacks with all other XP buffs!
now looks like scam - or bug - or both.
I'm not happy
Edit 2016-04-06 - Psijic now correctly stacks with the Cake - thank you @ZOS very much
Edited by Gargath on April 6, 2016 4:40PM
PC EU (PL): 14 characters. ESO player since 06.08.2015. Farkas finest quote: "Some people don't think I'm smart. Those people get my fist. But you, I like."