UPDATE 07/17/2026
After some experimentation and examining the fogftp.class.php file, I found that FOG calls ftp_rename to move files from the /image/dev/<mac_host> folder to the /image/<image_name> folder.
Finally, it helped me to connect with a regular ftp client (WinSCP) and try to move the folders manually. This is how I found out that the problem was in the permissions set on the folders.
So I found out that from my original version 1.5.10.10 to the current 1.5.10.1903 there was a change in the file
/lib/reg-task/taskqueue.class.php
in the private function _moveUpload() method it was previously
->chmod(0777, $dest)
and now it is
->chmod(0775, $dest)
That (maybe) could be the reason why it stopped working for me. Thanks to this post https://forums.fogproject.org/post/156271
chown -R fog:users /volume1/images/
#set inheritance to the dev folder
chmod g+s /volume1/images/dev/
I connected to the NAS with ssh and fixed the permissions over the folders. Now it’s working fine again.