Sorry to hear that. I suspect that this is a permissions issue, where you might not be able to modify the app-level object. Your ability to access or "read" the object might be the same, hence the working GET operation. It might be worth investigating with the group who administers your instance, or asking them to look into your permissions and how they might impact this.
My colleague suggested that, if possible, you might want to look into your own app-level permissions. You can also try to see if other users of the same app can edit the dashboard and what their user role might be, if different from yours.
This docs topic might be helpful for more details:
http://docs.splunk.com/Documentation/Splunk/6.5.2/Knowledge/Manageknowledgeobjectpermissions
In the meantime, you could try cloning the dashboard and keeping the new version "private" so that you continue to have read/write permissions and can modify it further.
Let us know if you make any discoveries!
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