原文：http://forums.esri.com/Thread.asp?c=93&f=989&t=289930
-----------------
I ran into this problem today and discovered that, for me, the solution was in how I opened the .mxt document that I wanted to modify.
Originally I was double clicking on the .mxt file in ArcCatalog,
making the changes, and then attempting to 'save as' and got the error
message above.
However, when I started with a blank ArcMap project and then used
File, Open, changed the type to .mxt, select the .mxt file I wanted to
modify, and made the changes I was then able to save it back to a .mxt
without any issue.
--------------
我遇到的情况是双击mxt用arcMap打开，不能保存，出现标题中的错误提示。但是先启动arcMap，再用菜单：file》open打开mxt就能保存
http://forums.esri.com/Thread.asp?c=93&f=989&t=289930