802 error - Unable to open file.
If the FileMaker Server doesn't have an SSL certificate, or you are accessing the server through an IP address or localhost this can cause FileMaker Go to refuse to connect to the file and it will exit the sync with "802 error - unable to open file"
You can easily test this, by navigating directly to the server from a mobile device that isn't working. Once you do that you should get a dialog warning you about the connection. Choose "always accept connections to this host". After that try the sync again. It should work now that you have told FileMaker Go that is ok to connect to this host.
The better solution is to get an SSL certificate installed and always connect through the secure domain.