I got the following Error in android Studio 2.2.3 when I sync gradle.
Error:Cause: unable to find valid certification path to requested target
I have faced this problem with Maven build earlier. It was occurring because I was working in a restricted office network. You need to import required certificate in your keystore.
Follow Steps in the answer of below question "unable to find valid certification path to requested target", but browser says it's OK
(use double quotes for fienames/pathnames)
Note: I am not able to find now exact source from where I solved problem, if I get it, I will update it