I took the risk to restore factory settings, not knowing whether that would fix the problem or not... After doing so it could connect to any free network but not mine which is wpa2 protected! So I followed the advice Trancedancer gave (rebooting the router) and now n900 connects like a charm to my router! I would come to this eventually, since I usually shut down my router at nights but you saved me from loads of speculations and reasoning... So Trancedancer, thumbs up!!!

So, problem solved for me, BUT I would really love to know if an alternative method (using the terminal) exists! Restoring the factory settings is a tad annoying afterall!!!