Shortcut icons have gray X's

My friend's new Sony laptop started having the icons on his desktop shortcuts change intermittantly. The usual curved arrow is on them most of the time, then they will change to a gray X for about 10 minutes, then to a green checkmark for a while, then back to the curved arrow. The only backup software he has installed is Carbonite, and the Carbonite support tech says the X's are not from Carbonite, which uses green and orange dots. (And I don't think Carbonite backs up shortcuts anyway). He does have Dropbox installed, but it is only set to monitor one folder. Any ideas?

Thanks