What does it mean when there's a red X on a file icon rather than a green tick?

It means theres an error. 

Mouse over the Dropbox icon on your status bar and see what it says. 

dropbox folder has green check mark, subfolder has green check mark, file in subfolder has red x

As said, it means their is an error with that file.

Oh really? That helped a lot! 

Mark already explained how to see what that error is.

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Mouse over the Dropbox icon on your status bar and see what it says. 

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He's talking about the Dropbox application icon in your system tray (Windows) or menu bar (Mac). NOT the Dropbox folder.

I just had that happen and I noticed the enclosing folder, which had the red x, had an emoji smiley face in the name.  I edited the folder name and removed the smiley face and it switched from a red x to a blue syncing icon and eventually to a green check.  Same with all of the files and folders within.

Do you have any special characters in the offending file or folder name?

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rob877 wrote:

Do you have any special characters in the offending file or folder name?

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Illegal characters is a common cause of that, as is a files that's in use or locked by another process. For illegal characters, you can use the Bad Files Check to list all files in your account that may have a problem syncing.

• https://help.dropbox.com/syncing-uploads/files-not-syncing
• https://www.dropbox.com/bad_files_check

I have received a red X on our drop box folder.

I cannot find any file with bad characters. This occured immediately after a major Drop Box update that took some time to sync on my computers.  However my main computer still has a red X and others do not.

This link will not load either:

https://www.dropbox.com/bad_files_check

Thank you for the very quick reply.

Hovering was the very first thing I did (Windows). All it says is that it is currently syncing, but it has been doing this since last week in this one particular computer