Paul Bertucci, a valued member of the MerusCase support team, sent this out at around 6pm tonight (with the private portions removed, of course):

Hi (Merus User), 

We noticed a few notifications of failed uploads/quicksaves with a file you were working on today, "Document Of Great Importance.docx". We did a little bit of digging, and the modified date on this file (when it was originally downloaded) is about a week old. We just wanted to let you know that, for security purposes, downloaded/merged documents in MerusCase have a security token "key" in them which expires after 72 hours.

If you've got a document that gets worked on intermittently over days or weeks, you should probably download it from activities when you resume working on it after a break, rather than continuing with the original downloaded file saved to your computer. Since you're QuickSaving regularly, you'll still have all your changes and shouldn't notice any difference.
Let me know if you have any other questions and I'll be happy to help.

 I'm pretty proud of that level of service.