ClassNotFoundException for SignInConfiguration when signing in with Google

I solved my problem by putting "App signing Key" to OAuth 2.0 Client ID SHA-1 finger print instead of signed key which was generated by using keytool. If you have checked "Google Play App Signing" in Google Play Console, the usual signed key will not work. Check out your App signing Key in Google Play Console>App signing>App signing certificate and then copy SHA-1 certificate fingerprint and paste to Google API console OAuth 2.0 Client ID. It worked for me!

I think this might be a red-herring. I had the exact same stack trace and have spent a ton of time trying to determine the cause. I came across a similar post while researching which indicated it might be a Samsung only exception that could be ignore (them inspecting intents in transit or something). Sure enough, I tried on an HTCOne device and the exception no longer appeared. I still had the same overall Google Auth problems, but at least I could move on to the real issue which (for me at least) appears to be SHA/Cert/Account/Signing related. Anyway - hope this helps (something to check on anyway)