Had the same issue with Acrobat DC. None of the fixes above worked, found the answer that worked for me in my situation. (Windows 8.1)
At some point I had changed the name of the user on the PC. While the name of the user was changed, the files on C: (my documents, downloads, etc.) were still under the old name. I went back into control panel and changed the user name back to the old name. Acrobat had no issues activating.
may not be the same situation as yours, good luck!