Originally Posted by naxa
This can be avoided if the input/source images are in grayscale.
I use XnView to batch-convert them; or more often a command line tool from xnview team called "nconvert.exe", like this:
nconvert.exe -grey 256 -overwrite *.jpg
This would convert all jpg images to grayscale by overwriting them.
So don't forget to back up your original images first!
(Note: I recommend nconvert only for users already familiar with the command line, or else doing a quick tutorial on command line first, before trying to use it. For the rest of the users I recommend doing it with XnView's batch conversion feature.)
It seems that mangize has a trouble with working on non-grayscale images sometimes. I used to get this very same error; since then, I pre-convert my source images to grayscale before starting to work with mangize, it started working again.
Hope it helps!