Starting with
Adblock Plus 3.6 we stopped supporting filter lists served over unencrypted HTTP (see
announcement) which may be why you can't add some filter lists. Apart from that, we simply check whether it's a proper Internet address.
Note that we're going to include a more descriptive error message as part of the upcoming Adblock Plus 3.8 update to specifically point out whenever that is the reason instead of showing the generic "Failed, not a valid address" error.