Don't know if this is a NWN bug or an EfU one. Anyways, I tried casting my Protection from Evil spell >_> while having Defensive Casting mode enabled. Didn't seem as it mattered if I was able to make the concentration roll or not, spell didn't go off.

Here's a screenie:

Happened to me with my wizard too.

I believe this is an issue with spells like protection from evil/the shadow variants -- anything where a single spell has multiple options, if that makes sense. Bioware issue.

As this is a Bioware issue, I don't believe that it's something that we can fix.