I experienced the same issue on the 3rd day of upgrading from update 4 => 6.1.3-15152 update 5
I was changing some package settings (Media Station) followed by accessing the Shared Folder application when a pop-up prompted an error about encrypted key not matching/missing and no access rights. I tried to access my DSM via another browser tab and received the error page "Sorry, the page you are looking for is not found".
SSH was up and dmesg also shows similar output to user @cremenescu's log.
md: Autodetecting RAID arrays.
md: invalid raid superblock magic on sda1
md: sda1 does not have a valid v0.90 superblock, not importing!
md: invalid raid superblock magic on sda2
md: sda2 does not have a valid v0.90 superblock, not importing!
(... etc, until sda6)
Recovery
Booted with a newly created DS3617xs (1.02b) boot loader, Synology Asistant shows status as "Recoverable". Attempting recovery (which completed with a reboot) did not help as the page still shows "Sorry, the page you are looking for is not found".
I was only to recover my volumes after seeing @cremenescu's method of using the a different DS3615xs boot loader. Doing so allowed Synology Assistant to see the NAS status as "Migratable". I proceeded to do so and DSM was functioning again. All my files and settings were intact. I manually updated to 6.1.3-15152 update 4 instead after the migration as a safety measure. My System Details (before issue occurred)
Intel NM70 chipset (Baremetal)
Celeron 1037U @ 1.80GHz / 4GB ram
IOCrest 4-port Sata Controller using Mavell 88SE9215 chipset
Boot: jun's boot loader 1.02b / DS3617xs
I cannot ascertain that my issue is due to update 5, but I hope by sharing what I experienced will be of use to the community.