The DMG contains spell scrolls (on page 200), with the following rule:
If the spell is on your class's spell list, you can use an action to read the scroll and cast its spell without having to provide any of the spell's components.
This is all well and good for spells with a casting time of 1 action (which admittedly is most of them), but for spells with a reaction casting time, how is it supposed to work? Especially since reaction spells have specific triggers that allow you to cast them.
In case it needs saying, I'm not interested in speculation here – only RAW-based answers or explicit statements of developer intent should apply.
Best Answer
Spell scrolls have received errata, which replaces the sentence given in the question with:
So a spell scroll that holds a spell that requires a reaction to cast now requires a reaction to cast, resolving the contradiction.