[Unable to extract messages from stored DBX files]

You did all the correct things for backup and restore.  Make sure to copy the DBX back from USB Flash Drive to hard drive, make sure the files are not write protected.  Right click on each file and select Properties. Uncheck Read Only.
If no joy, do you have any backup DBX files to use? If YES, then try to restore  files from backup.  These seemed to be corrupted beyond repair. If NO, try a better repair tool - OE-Mail Recovery - free trial version. http://www.oemailrecovery.com/dbx-extract.html

[Unable to extract messages from stored DBX files]

Look at this information also on this reputable source. 

http://www.reddit.com/r/Database/comments/2c4hcw/how_to_fix_repair_outlook_express_folders_and/