Health data export remains in preparation and cancels itself

I am using an iPhone 6 Plus with iOS 11.1 and I need to export my Health and Activity data. I do so using the dedicated function but the preparation operation lasts quite long (up to a few hours) before cancelling itself. Normally I am expecting it to propose different means of export (email, message,...). But the process does not get to that stage. The operation does not throw any error or warning message.