May I repeat here what I posted in a recent proxy thread?
My Samsung Galaxy Note 2 just updated to Android 4.1.2 and yes, this appeared to break Adblock Plus.
After some searching I found that you need to enter the correct proxy data for each one of the wifi networks you use. The proxy settings are hidden under settings -> WiFi -> [the network's name].
I found the following instructions online that worked for me:
Go to Settings > Wireless & networks. Long press on the name of the network that you need to add the proxy information to. When the popup box opens tap Modify Network and a new box will open. On the bottom left there is a check box that you need to check labeled Show advanced options. That will then reveal the Proxy settings and the IP settings options. Change the Proxy drop down from None to Manual. You will then need to scroll down a little bit and enter in your proxy information.
For Adblock Plus you need to fill in "localhost" and in the next screen (for port number) "26571".
Since I did this, everything seems to be working again as it used to.