If this issue persists, try checking the image size to compare to the canvas size. I had the same issue and found this to help. Make sure the image size is not something like 20,000 inches. Try making it smaller or changing the measurement to pixels, if it is not already. Next, make sure the canvas size is not smaller than the image size, nor bigger than it. If so, change it to the same size with the same measurement of pixels or whatever the image's is. Try saving for web, again. This should work, if not then try removing the profile files (Users/[Username]/Library/Preferences/Adobe Save For Web 12.0 Prefs). Then restart your Mac and relaunch photoshop. Hope this helped!
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