When attempting to Photomerge from Lightroom to Photoshop, the images get stitched together fine (I can see them all together in the preview), but before I can do anything I get an error code where PS "unexpectedly stops working" and I have to shut it down. Oddly, 2 pictures can be merged together - only at 3 or more do I have this error.
I can only guess at this point. It could be related to other software that [might] be running. I wonder what would happen if you converted to JPEG. Probably nothing, just a thought. I am about ready to shut down for the night. Hopefully someone else will be able to help you. These things can be terribly frustrating, I know.