Today the problem suddenly reappeared. I checked IPConfig right away and the mask was back to 255.0.0.0 Had to perform release/renew routine again.
I have a suspicion that it has something to do with me saving the router configuration using that computer (through http://192.168.1.1)The router itself (BEFW11S4 ver. 3) has a correct mask. The LinkSys wireless card does not seem to have mask setting at all.
Though I know now how to fix it the root cause still remains a mystery to me.