For spells that require a powers check when cast in Ravenloft, when should the check happen for scrolls: during creation, at the time of the scroll's use, or both?
Using scrolls does require a Powers Check.
Page 118 from Realms of Terror (the Red Box Set):
"If a magical item imitates a spell, the item undergoes the same changes that the spell does (the Dungeon Master should consult Chapter VI for details)."
The requirement of a Powers Check is one of the changes to spells listed in that chapter.
2nd edition doesn't specify whether creating scrolls requires a powers check, but 3rd edition does.
The 2e rules for creating magic items in Ravenloft just state that the occurance is much rarer. The rules for scribing scrolls doesn't describe the creation processes as actually casting the spell, so RAW nothing in scroll scribing triggers a powers check.
The 3e Ravenloft campaign book does specify that creating evil magical items in Ravenloft (including potions and scrolls) requires a powers check that is even more difficult than the one for just casting the spell. The Powers consider allowing the spread of these magics through items to be even more offensive.