The following situation happened twice in my games tonight. A pop up window with a required selection was triggered (advisor and druid) then/while another object was viewed (right clicking on a kingdom card or being in kingdom view). In both cases, when returning to the pop up window, it was impossible to make a selection. The command bar said "choose x", and the pop up window was there showing the cards, but hovering over or clicking on the cards did not do anything. In both cases, we had to undo the action and reactivate the pop-up window in order to allow a selection.
I know this can happen but I don't know what causes it :(
What you can try is to reload the browser windows (CTRL-R). That hopefully solves the problem without an undo.