After unzipping the file to get to your *.DEB version you have to rename that version. So lets say you are on 3.3.02.165 and you want to go back to 03.03.01.156, do the following.
Download gpl-source-wd_my_cloud-03.03.01-156.zip
Unpack the zip file to the *.DEB file.
Rename the *.DEB file to a version number that is highr than the version you are on. Example: rename 03.03.01.156 to 3.3.02.166 to replace 3.3.02.165.
Go to your Dashboard and do a manual firmware update. This will restore the older firmware.
BEWARE : that any scripts/functions that have been changed with the firmware you are on and are not addressed with the older firmware are still the current version’s script. I would do this as a last resort in trying to find a fix elsewhere on the forum.
After a few days other problems occured with this new firmware 4.00.00-607
It took me hours to find out which were the datasets causing the problem (pdf’ and wpl’ files are certainly causing problems during the scanning process) but at last i decided to downgrade the firmware to v03.04.01-230