Indeed the message does not provide the most useful information about the problem and we will try to improve it in the future. In some scenarios we show this error when the uploading process throws time out error and we encourage the user to manually check whether the build package has arrived at iTunes console.
However I am glad that you have solved your issue. In case it happens again I will ask you to open a private support ticket
where a further guidance will be provided.
Please excuse us for any inconvenience caused.