I would second Wendel's question - is the file there when you navigate into the Imported Items folder?
What happens with the server and the drive that the data is on overnight? Does it stay on? Are you running a backup other than the AC backup? Are you having problems with any other app/data on that machine?
Are the folders that AC creates within the Imported Items directory there the next day?
Just trying to get more details to try and help you diagnose.
