Apparently, a few files included in the update process contain the word "sex" and are thus not able to obtain the files while updating. I kid you not.
VarusExpirationTimer.luaobj
and
XerathMageChainsExtended.luaobj
and
XerathMageChainsExtended.luaobj
ISPs are blocking any URLs that contain specific words that typically lead to porn. How do you fix it? Either get a friend that doesn't have this problem to share their League of Legends folder with you after every update, or contact your ISP to remove the filter on your account.
This is just another in a long string of legitimate services and sites that are being blocked by the UK's anti-porn crusade.
...right?
I'm sure this is mostly to stop children getting their mouse cursors on it without their parents knowing, at which point you should probably become a better parent, instead of having your government take everyone's freedoms away.