Philip,
No real progress, I'm afraid... I've tried so many times by now that I have a total of about 50GB of upload trafic this month

. I still can't figure out what's causing the problem: I start uploading the 1GB file to WeTransfer and that should take 2 hrs, but after the 2 hrs it says: "One minute left" and a couple of second later it just starts all over again... I've left it uploading during the day, at night, in the evening,... but nothing works. I'm uploading (again) right now and am monitoring my upload/download speed with a small tool called "DU Meter" in the hope to see the glitch...
I had no problems before, uploading larger files to WeTransfer and I initially thought it had to do with WeTransfer, but I'm starting to think it has to do with my internet connection or ISP, although I have enough upload/download capacity available and I never had problems before...
I'll keep on trying and keep you informed (I'm on skiing holidays next week, starting this Friday, so it might take a while).
Maarten