In my case, closing every ms app, killing the process called "Office365ServiceV2", and deleting all the files with names that starts with letter "c" or "e" under "~/Library/Group\ Containers/UBF8T346G9.Office" solved the problem.
So run this (after closing all of the office apps, including onenote):
cd ~/Library/Group\ Containers/UBF8T346G9.Office
ls -a | perl -n -e 'print if m/^[e|c]\w/' | xargs rm
... and try to active again.
Hope this will solve the activation problem for you too!
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