We have used your plugin happily on a client's website for a long time, but recently there was a problem.
Dozens of posts were set to expire last month, but we went in and edited them to change the expiration date to a future date. When the date they were originally set to expire came along, around 3 quarters of the posts we changed did actually expire, even though when editing the post it still shows the future expiration date.
So even though the post expiration date has been saved, it still expired on the original date.
Do you have any idea what could be causing this? Here is a link to the logs: http://pastebin.com/2LXKTiFs