How did I do it? Well, I just don't know. Just by curiosity I turned on my backup N900, and there downloading is working as well again.
I worked with Windows environments for 20 years as a programmer etc., and normally I can deduct the origin of problems quite well. This problem however, coming out of nowhere and disappearing again, puzzles me completely.

I must say the message I got (the HTML I downloaded) "<addinfourl at 1102820816 whose fp = <socket_fileobject object at 0x41bb8d70>>" looks a lot like an IP socket problem. Problems like these happen every once in a while in my Visual Basic programmes as well. Mysterious stuff, never could trace the origin of those.