There are at least three threads in iStocks 'Help' forum on this - they are still looking into it.
Someone posted this as a workaround - can't confirm as haven't tried it:
"you actually can still add the three categories, it's just a little trickier than it was before. after you select your first category, you then try to enter a second one, you get an error message (as everyone now knows). however, in the selection box on the right, you will actually now notice that there is a scroll bar moved all the way to the right. if you move it back to the left, you can then make your selection for the second category. do the same thing for category three and you are in business. i've done this for all 15 pictures i uploaded yesterday and today without any troubles."