Syncing Problem: When error message doesn't apply?

Hi,

I have a folder of 18 files that won't sync. When I clicked the icon for more information, it said it was because the title had a character that wasn't allowed. However, it doesn't have any such character! The file name has letters and underscores, and then the extension. I tried removing the underscores just in case. It still wouldn't sync. Any idea what's going on?