Latest Adblock Plus extension installed in Chrome and Firefox browsers has broken YouTube and Dailymotion sites today, Malware Domains filter has caused this. So you just need to disable it to use Adblock Plus on YouTube and other sites in browsers.
Generally, after installing in Adblock Plus, its first run page offers options to block malware, remove social media buttons, and disable tracking, if you’ve enabled Malware blocking aka malware domains filter, you need to visit ABP preferences, and uncheck Malware domains filter in the Filter lists in affected browsers.
ABP team is aware of this issue and is working to find a solution, meanwhile, suggests users to disable Malware Domains filter to continue using their add-on.
Another adblocking extension ublock origin is working fine without any issues
Also see: What is Adblock Warning Removal List?
UPDATE January 16, 2016: Update the Malware Domains filters manually now to fix the problem, or wait for the ABP update to arrive and resolve.
Accidental block of ‘s.ytimg.com’ site (this site was used by YouTube to display content) in yesterday’s update for Malware Domains filter has caused this. This site has been removed from MD block list, update Malware Domains filter now.