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Did you try downsizing the image drastically - say, to one tenth the size and then merging the downsized images just to see if that is successful?Though not a pleasant thought you can downsize you image by 2X and probably have a pretty complete workaround.
If the downsized images merge well, it would prove to some degree of certainty that:Can you elaborate on what new, useful information would come from that?
Assuming merging process works with one tenth the sizes, assuming it still works at one fifth, if it does not work with one fourth - then 'something' is facing difficulty with merging that particular sized images (2125 X 3500) and above but not below that size.I can certainly turn normal-sized images into smaller ones and keep shrinking them until Photoshop is successful, but will it tell me anything valuable?