1434 - Error: "The filename, directory name, or volume label syntax is incorrect"

Symptom

A backup completes with the error "The filename, directory name, or volume label syntax is incorrect".

ERROR : 19:18:06 Failed to add '[file path]' (The filename, directory name, or volume label syntax is incorrect)

Cause

This error indicates that the specified file or folder name contains one or more illegal characters. A character like this is usually represented by a question mark (?) in the logs, but in the file/folder name it may be a pictograph, square, or similar. Certain characters that are not supported by Windows can cause issues with backups. 

Solution 

To enable backups to proceed smoothly, rename the file/folder in question, or delete it if it is not needed. To avoid encountering this error again, be sure to investigate how the problematic file/folder was created.