One thing I have had happen to myself is where I have downloaded a giveaway - and left it for several hours to activate (sometimes I have downloaded in the first 60 minutes of the giveaway and don't get around to activating until the final 60 minutes).
Sometimes during the course of a giveaway the GOTD team update the download with a new version/build. The side effect of trying to activate a "previous" release of the giveaway is the mysterious "giveaway is over" error message - when you know there is plenty of time left (even the website says so).
The "real problem" here is an inaccurate and misleading error message.
If this happens - the first thing I would suggest is to redownload and activate immediately.
It is entirely possible there are other causes - but this one I have found to happen to me - and it is repeatable. It is obvious to see the downloaded files have changed are different - if only from the file sizes.
(BTW - The GOTD Team are aware of this issue now, and have told me they plan to address it).