Well how are you saving the file? Are you using Save As? or the Export tool. Maybe you are setting the compression too high. Or you changed the Palette it can use to something else, Giveas us a few more details and maybe we can narrow the problem down.
As it is now, there's not much more I can say.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.