OK, so I just had this happen and after cursing the development team for this bug (I'm running into quite a few bugs, I find, but nothing this serious), I figured out the solution, and it's pretty much what someone else said, but I'll give more details....
For me, I have a theory, although unsure if it is true. Tonight, I changed my throne. I went and did a bunch of quests, one of them the Thom Rainier stuff. I got to the point where I had to judge him. I went to click my throne, it clicked, then it loaded for a long while, and came back to what I call a BROKEN throne. A broken throne would be where you can see the diamond for the quest there, but you don't get the cursor to click on the throne to do it. My theory is if you change your throne maybe while you have judgements to do, it will break the throne.
Anyways, more importantly... how I fixed it for me...
1) Ensure your throne is NOT in the broken state. By that, I mean, you need to be able to click on it (don't click on it, however). You should get your cursor thing when you hover it. If you don't, then your throne is "broken". Launch a save that will allow you to hover your mouse over the throne and get the cursor but again, DO NOT click).
2) Go to the undercroft and change your throne. If you changed your throne in the past, change it back to whatever it last was (just a theory on my part)
3) Now, go do your judgement
Hope that works for people. Thumbs down for this bug being known for half a year and still not fixed!!