Sending data to a webserver with GPRS/GPS Shield (U-blox Sara G-350)
I worked on a project similar to yours. I think you forgot to establish a connection first.
Just add those lines of code at the top of setup() method:

 gprsSerial.println("AT+UPSDA=2,0"); //Reset connection
 toSerial();
 delay(2000);
  gprsSerial.println("AT+UPSD=2,1,\"weborange\""); //Establish a new one
 toSerial();
 delay(2000);
 gprsSerial.println("AT+UPSDA=2,3"); 
 toSerial();
 delay(2000);
 gprsSerial.println("AT+UPSND=2,0");
 toSerial();
 delay(2000);


I hope it works for you. Else, you can use AT+UHTTPER=2 after all to check for error source.