webRequest constantly failing on 400 on some devices while working reliably on another ones

I read Google Calendar to show it on the watch face. 

It works as a charm on some devices while constantly fails on another ones. Mostly iPhones.  

The next request returns error code 400 with data == null:

Communications.makeWebRequest("https://oauth2.googleapis.com/token", {"client_id"=>"934…alc.apps.googleusercontent.com", "client_secret"=>"mI…nY","code"=>"AH…KKw", "grant_type"=>"http://oauth.net/grant_type/device/1.0"}, {:method => Communications.HTTP_REQUEST_METHOD_POST}, method(:onTokenRefresh2GetData));
What could be wrong? 
Thanks