When wielding a Rapier in my main hand and nothing in my off-hand, I am unable to cast the Shield spell on my Shadowcaster Rogue (High Elf, Level 5 and 6). This took place in the Dark Castle. I have not been able to test it anywhere else, or with any other weapon. It is a fresh save.
One thing I noticed is that this began happening the first time I elected not to cast Shield when given the option, and then elected to use Uncanny Dodge to reduce the damage I took.