I uninstalled the game, deleted the Program Files Academagia directory, added DLC 16 again, and now everything seems to work. Don't know what the problem was, but I'm now able to overwrite files again, so it looks like it might be fixed. I'll let you all know if it crops up again, but thank you for your help; I think it might've been when I put in DLC 16 for the first time, since that's when it started happening. Thanks for your time.