Ring of Arcane Stillness: characters who can use it via UMD get the feat in their list, but it does not work (spells still fail from armor penalty in tier 1-3).
A few queries about this-
What class are you casting the spells from?
Spells themselves or just scrolls?
Are you wearing items that have arcane spell failure as an item property or only from armour and shields' innate properties?
Were you trying spells while exhausted?
'Cause theres a few things that would need to be checked in regards to what could override a feat that by default in NWN, does actually work properly, its not something that is so easily broken so it really needs more in depth details.