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I ran into the same issue and none of the 'solutions' posted in this thread have helped. After lots of testing, here is my conclusion.

The original editable PDF is created with Adobe Professional (with Extend Features in Adobe Reader enabled). This PDF is then emailed to various people. As long as each user opens and edits this PDF in Adobe Reader (NOT PRO), the changes are saved and the next person can continue to open/edit without any issues. If someone opens this PDF in Adobe Pro, and then saves the file, the PDF then loses the Extended Features. From that point on anyone who opens this PDF with Adobe Reader will get the error message:

"This document contained certain rights to enable special features in  Adobe Reader. The document has been changed since it was created and  these rights are no longer valid. Please contact the author for the  original version of this document."

Only individuals with Adobe Pro will be able to open this new PDF and make edits/saves.

Long story short, once this PDF leaves the original creator, make sure everyone who opens/edits/saves this file does so with Adobe Reader (the free version from adobe.com). Or have everyone upgrade to Adobe Pro, which might be a slightly more expensive solution.