I just stumbled upon the fact that wearing Medium Armor (15-34.9s total equipment weight) lowers spellcasting success chance, and Heavy Armor (35s+ total equipment weight) lowers it even more.
Do any webpages here go into more detail about this? I didn't even realize it (despite having played this for years) until I coincidentally noticed my spell success chance fluctuating.
(Though I now found out it's mentioned in-game in Help -> Equipment Weight, woops)