This error indicates that the FTP put command failed to write the dump file to your remote FTP location. This may be a result of lack of disk space in the remote destination ftp folder, it may also be lack of write permissions issue with the username you specified in the control panel for the FTP action.
To recover the error, examine your destination FTP folder and make certain it has ample disk space. To confirm your username has proper write permissions you can use an FTP browser (such as WinSCP), log into that remote location and confirm the permissions are adequate.
The dump file that's created can not be extracted or used. The dump file is created locally before transferred to the storage location by FTP. However, Agile Central On-Prem is immediately deleting this file after the backup. This is part of a cleanup process that ensure enough disk space on the On-Prem server. The dump file will be deleted, can not and should not be used.
To recover the above error, make certain you fix the Access problem to the FTP site, then perform the backup again.