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 had the same problem. I fixed it by removing/commenting proxy settings in gradle.properties. Check your gradle.properties, if you have some like this
systemProp.https.proxyPort=8080
systemProp.http.proxyHost=192.168.1.1
systemProp.https.proxyHost=192.168.1.1
systemProp.http.proxyPort=8080
comment it with #. Which will look like this
#systemProp.https.proxyPort=8080
#systemProp.http.proxyHost=192.168.1.1
#systemProp.https.proxyHost=192.168.1.1
#systemProp.http.proxyPort=8080