I did the upgrade from 6.2.3u3 to 6.2.4 like a noob.
On the morning I received the mail from Syno telling me there is an update available.
I checked the ReleaseNotes, meh it's just a minor upgrade let's do it.
Totally forgot to check the forum before applying the upgrade.
Thank you so much @IG-88 for your guide on how to downgrade to 6.2.3u3 !
The links you provide to other guides made your explanations really complete.
I personally do not see how to make it more clear.
I just had a doubt about deleting this file :
It was not present on my disk but there was file named SynoUpgradeIndexAAA.txz (AAA = something I don't remembre -_-')
I juste assumed it was the same and deleted it ending with no more file named /mnt/Syno...
Thanks again, you made my DS918+ back online without loosing data or configuration