I contacted the support line and was informed that Mendix is aware of the issue with this particular module and that a fix for it will not likely be rolled out until tomorrow, Monday September 6th.
Did you check status.mendix.com? Make sure that it is not down.
Another reason might be the content of the spreadsheet that you try to upload. Start with a simple one, then try a more complex one.
I've got the same issue. It happends during the creation of the app, before we are even asked to upload a spreadsheet.