There is some kind of bug that is not letting me put a bounty on a particular question. I have enough rep, the essential waiting period has passed and I have no other bounty currently running.
I have not encountered such a problem before, so I have no idea what caused it. I had tried in the morning also, but it didn't work. I have put bounties before under same conditions (Windows 7, Google Chrome latest version) on Puzzling and on Math. (And I don't think I have a bad connection.)
I've uploaded screenshots here.