@mjsutterer this is one we have not heard of before. If it looks like your network connection is OK, please contact us via firstname.lastname@example.org with some screenshots and preferably output from /Applications/Utilities/Console.app when you try to activate? To see messages related to Manuscripts in Console, you can filter for messages that note the word "Manuscripts".
If anybody else has experienced the same issue, please do note here as well. I am requesting contacting via email@example.com really just because you may care about the privacy for your system logs etc material that we may ask to help diagnose the problem with your license.
CEO & Co-founder of Manuscripts.app Ltd