I attempted just now (several times, with and without various "refreshes") to place a Bounty on this question from History.StackExchange. The Bounty dialog appears, but when I select an amount of 50 rep, neither the Next or cancel controls has any affect. The X-close control does work, and the Next button does go a darker shade of orange when I click it.
Same behaviour occurs if I select a bounty of 100-rep, so it's not just that I must specify a larger amount.
Does anyone know what I might be doing wrong? I have not placed a bounty on a question before.