I cannot upload the proof of purchase: the page hangs.
I have tried on the phone, but I did not know how to select a file, so I try on the PC, where I did select a file, but then the page does not want to continue. There is no error message, no time-out, just endless waiting. (well, about one hour)
Thanks to you all for your replies.
I have just been able to register my FP4. The upload of the proof of purchase has worked and I have received the warranty extension.