This error is often encountered when using the user to log in to FTP after we have restricted the user from jumping out of his/her home directory:
 

500 OOPS: vsftpd: refusing to run with writable root inside chroot ()

 
This problem occurs in the latest update due to the following update:
 

- Add stronger checks for the configuration error of running with a writeable root directory inside a chroot(). This may bite people who carelessly turned on chroot_local_user but such is life.

 
Since 2.3.5, VSFTPD has enhanced security checks so that if a user is restricted to his or her home directory, the user’s home directory can no longer have write permissions! If the check finds that write permissions are still available, the error is reported.
To fix this error, remove write permissions from the user’s home directory with the command chmod A-w /home/user, and replace the directory with your own. Or you can add one of the following two items to the VSFTPD profile:
allow_writeable_chroot=YES