supl.nokia.com is dead, just like nokia itself. Change the value to:

But I still can't figure out what is it all about? Mine n900 got fix under a minute everytime, with no cellular data, only gps, even when I only stick it through the window.

With google supl it's doing the same, mostly under 5 seconds.

But yes, my previous n900 had problems with fixing, no matter if cellular data was enabled. It got partially broken stick on motherboard and broken antena (After hitting the ground really hard).