Several possible reasons.
You could be trying to "POST" (Create) a list that already exists. If you are modifying the list you should "PUT" (Modify) it back.
You could be trying to make a change so that the list would be invalid. I.e. perhaps renaming the list to be the same as another list in the system.
Are you sure you are not conflicting w/an existing list?
As a test, try to GET a specific list, and then PUT the XML back w/out modification. Does this work?
Now try changing just one small thing and see what you get.
Tom M
Group Product Manager – Content Editing
Constant Contact