Restricted trade

This is a weird one.
Since the introduction of the new trade system, I can only ever use 1 free trade slot. I cannot access the other 3.
When I'm on the selling screen, there is what appears to be a slider bar (with no slider), flanked by two arrows, left and right, which do nothing.
Just above this 'bar', there is the legend "1/4", and the 1 is permanently darker than the rest, as if it is 'selected', but I cannot overtype it, or make it do anything useful. Clicking upon this spot shows there is some 'element' there, but I can do nothing with it.
Having checked with other players, I appear to be missing the slider to activate the other slots.
I have tried this with Firefox, Chrome, and even infected my comp with IE, all behave the same.
Nor is it server specific, the same behaviour manifests itself on all 3 English servers, my French, german and Testing accounts.
I'm waiting until I can try with my laptop, in case this is some manifestation of the game conflicting with some particular hardware, and then can also try Linux, to eliminate some bizarre Windoze bug, but in the meantime, this is incredibly restrictive, being only able to trade once every 12 hours.

The slider only works with resources. It doesn't work with adventures or deposits.