This is unusual, and I suggest to provide a link to a complete kodi.log.
To rule out a hardware defect, you can also try a different OS, like Raspbian.
Is this an installation with added/changed settings in the config.txt file. Try starting up with the default config.txt file.
Also, try a fresh install anyway with the default English language setting (I know, you French, hehe...), but it could be a language setting problem.
Thanks for your help 
I already tried to start from fresh install on brand new SD card, but problem is same : ok for first boot, but if I shutdown my system, it doesn't start : stuck at "Librelec 9.2.6 (official)"... 
My config.txt is the same as original :
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I believe that I am having the same problem. It's a brand new RPi4 2GB model and a fresh Libreelec 9.2.6 install. Occassionally, it seems to boot up fine. But most of the time it appears to hang at the step saying "Reached target System Time Synchronized." It isn't actually hung, because I can still SSH in to it and get a command prompt or access the files.
I've attached a kodi_logs.zip with 4 different kodi.log files. 1 from a successful boot and 3 from failed ones.
Also, when it is "hung", if I connect and issue a reboot command, it shows 3 services running a stop job. One is kodi and that one usually completes. The other 2 are: Argon One Fan and Button Service and Broadcom sdio firmware update for BCM43430A1. Those alternate for a while. Then the Argon One service stop completes and it goes through stopping more services. Then after the Stopped Login Service, It starts showing the "A stop job is running for Broadcom sdio firmware..." again. Eventually that seems to complete or timeout and it proceeds to "Reached target Reboot." It doesn't actually reboot at that point. And if I power off the system and power it back on, the "hang" just repeats.
Nobody ? 
I have exactly the same problem. I'm using a Raspberry Pi 4B (4GB) with an e-marked USB-C 18W power supply and a wired Internet connection.
I would try to boot from an USB Flash key (or USB/SATA adapter) instead of SD card.
Make sure you have a fresh bootloader / VL805 firmware installed, you may also need to tune up the bootloader options.
Read carefully here (including all links published there): Raspberry Pi 4 libreelec not booting
I installed Kodi 18.7 on Raspberry Pi OS and it is working very well. It seems to be a problem with LibreELEC, not Kodi?
Maybe similar problem here, but only occasionally. Most of the time, it works flawless, but sometimes I get this:
[Time] Timed out waiting for device /dev/serial1
[Depend] failed for Broadcom sdio firmware update for BCM43430A1
[Failed] Failed to start Load Kernel Modules.
See 'systemctl status systemd-modules-load.service' for details.
Mounting Kernel Configuration File System...
Starting Apply Kernel Variables...
[OK] Mounted Kernel Configuration File System.
[OK] Started to create list of required static device nodes for the current kernel.
[Failed] Failed to start Apply Kernel Variables.
See 'systemctl status systemd-modules-load.service' for details.
[***] (3 of 4) A start job is running for wait for Kernel Time Synchronized .
Startup works for weeks, then this is happening several times in a row. Trying again later works again. Any ideas? Maybe a thermal issue?
This problem seems to be solved after I upgraded LibreELEC to 10.0 Beta.
Same problem here, brand new 9.2.6 image on a Pi 4 2GB with bootloader v2021-04-29 and it just won't boot. Hangs at a black screen with a frozen cursor on the top left corner. Pi OS works, RetroPie works. LE10 beta works but is unusable for me (no wired ethernet connection, CEC stops working randomly, it's in the family room, I need it to just work so beta software is not a good fit there)
Same problem here for the 3rd time:
Just tried with 9.2.7 and it's the same behaviour.
