Foamdad: it would not let me cast Searing Light. I could memorize it to my spellbook, but it was greyed out, even after resting, and I could not cast it. This also happens with other spells such as summon monster and negative energy ray.
[,...] It really seems like a bug.
Not a bug. I know what you're doing wrong. You're right-clicking your own character (or the ground, or an object, in the case of searing light) when you access the radial menu, or using the radial menu hotkey.
Searing Light is a spell that can only be cast on enemies. (Or allies, but don't.) If you right-click yourself, or the ground, and try to select the spell from the radial menu, it will of course be greyed out because you can't cast it on those things. But if you right-click on a goblin and select the spell -then-, it should work fine.
Summon Monster is a spell that can only be cast on the ground. If you right-click on a character or use the radial menu hotkey, you can't cast it.
There's an easy solution for this problem: don't use the radial menu. You can use it if you really want to, and when you right-click in the right spot it will work fine, but it's a pain in the butt. You need to click so many times.
Drag spells from your spellbook to your quickslots instead. You have 36 of those for a reason. Much easier and it avoids targeting shenanigans.