Just some random thoughts...
Are you operating on the same kind of image (e.g., RGB color vs. grayscale)?
Do you have the proper layer selected when you start the action?
In the Adjustments panel fly-out menu, are the Add Mask by Default and Clip to Layer settings as they were when you recorded the action?
By any chance is the curves an adjust layer and the document has layers and perhaps an empty or near empty layer. If your action and document meet those condirions. Use your adjustment Panel Fly-out menu and make sure the item Clip to Layer is not checked, This will cause action the recorded adding adjustment layers applied to all layers to be clipped when the should not be. An adjustment layer clipped to an empty or near empty layer is likely to show nothinf in the adjustment dialog like curves, levels etc.
Note: Action step make adjustment layer does not have with clipping mask but curve is clipped. Clipped to the empty layer I added for healing.
Not new to CS6 its in CS4 and CS5 as well...
Also note missing Layer Mask on the adjustment layer un-checking Add Layer Mask by Default in the Adjustment Panel Fly-out will also cause that error in old actions that worked.
So make sure Add Layer Mask by Default IS checked and Clip to Layer IS NOT checked if you want actions to play as recorded. Don't try to change Photosho default setting have Photoshop behave like it did before CS4 and work. CS6 has many other problems also. I had to stop using it...