mysql.service: Main process exited, code=exited, status=1/FAILURE - mysql

Question

i cannot start my mysql server, it crashed all a sudden and i can not find any useful log to say why.
my disk has still some space left, several Gigabytes.
last logs from journalctl -feu mysql are like this, which doesnt help at all:
May 06 16:04:23 soy systemd[1]: Starting MySQL Community Server...
May 06 16:04:23 soy su[5927]: Successful su for mysql by root
May 06 16:04:23 soy su[5927]: + ??? root:mysql
May 06 16:04:23 soy su[5927]: pam_unix(su:session): session opened for user mysql by (uid=0)
May 06 16:04:23 soy su[5927]: pam_unix(su:session): session closed for user mysql
May 06 16:04:24 soy systemd[1]: mysql.service: Main process exited, code=exited, status=1/FAILURE
May 06 16:04:24 soy systemd[1]: mysql.service: Failed with result 'exit-code'. May 06 16:04:24 soy systemd[1]: Failed to start MySQL Community Server.
mysql version 8
mysql Ver 8.0.20 for Linux on x86_64 (MySQL Community Server - GPL)
Linux soy 4.15.0-65-generic #74-Ubuntu SMP Tue Sep 17 17:06:04 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.4 LTS
Release: 18.04
Codename: bionic
and here the service info:
service mysql status
● mysql.service - MySQL Community Server Loaded: loaded (/lib/systemd/system/mysql.service; enabled; vendor preset: enabled) Active: failed (Result: exit-code) since Wed 2020-05-06 16:04:24 UTC; 7min ago
Docs: man:mysqld(8)
http://dev.mysql.com/doc/refman/en/using-systemd.html Process: 5961 ExecStart=/usr/sbin/mysqld (code=exited, status=1/FAILURE) Process: 5876 ExecStartPre=/usr/share/mysql-8.0/mysql-systemd-start pre (code=exited, status=0/SUCCESS) Main PID: 5961 (code=exited, status=1/FAILURE) Status: "Server startup in progress"
May 06 16:04:23 soy systemd[1]: Starting MySQL Community Server...
May 06 16:04:23 soy su[5927]: Successful su for mysql by root
May 06 16:04:23 soy su[5927]: + ??? root:mysql
May 06 16:04:23 soy su[5927]: pam_unix(su:session): session opened for user mysql by (uid=0)
May 06 16:04:23 soy su[5927]: pam_unix(su:session): session closed for user mysql
May 06 16:04:24 soy systemd[1]: mysql.service: Main process exited, code=exited, status=1/FAILURE
May 06 16:04:24 soy systemd[1]: mysql.service: Failed with result 'exit-code'.
May 06 16:04:24 soy systemd[1]: Failed to start MySQL Community Server.
error log:
020-05-07T07:55:44.761003Z 0 [System] [MY-013169] [Server]
/usr/sbin/mysqld (mysqld 8.0.20) initializing of server in progress as
process 2024 2020-05-07T07:55:44.762802Z 0 [ERROR] [MY-010457]
[Server] --initialize specified but the data directory has files in
it. Aborting. 2020-05-07T07:55:44.762827Z 0 [ERROR] [MY-013236]
[Server] The designated data directory /var/lib/mysql/ is unusable.
You can remove all files that the server added to it.
2020-05-07T07:55:44.762887Z 0 [ERROR] [MY-010119] [Server] Aborting
2020-05-07T07:55:44.762984Z 0 [System] [MY-010910] [Server]
/usr/sbin/mysqld: Shutdown complete (mysqld 8.0.20) MySQL Community
Server - GPL. 2020-05-07T07:55:45.026958Z 0 [System] [MY-010116]
[Server] /usr/sbin/mysqld (mysqld 8.0.20) starting as process 2032
2020-05-07T07:55:45.041675Z 1 [ERROR] [MY-011011] [Server] Failed to
find valid data directory. 2020-05-07T07:55:45.041828Z 0 [ERROR]
[MY-010020] [Server] Data Dictionary initialization failed.
2020-05-07T07:55:45.041889Z 0 [ERROR] [MY-010119] [Server] Aborting
2020-05-07T07:55:45.042277Z 0 [System] [MY-010910] [Server]
/usr/sbin/mysqld: Shutdown complete (mysqld 8.0.20) MySQL Community
Server - GPL. 2020-05-07T07:55:45.298208Z 0 [System] [MY-010116]
[Server] /usr/sbin/mysqld (mysqld 8.0.20) starting as process 2040
2020-05-07T07:55:45.305008Z 1 [ERROR] [MY-011011] [Server] Failed to
find valid data directory. 2020-05-07T07:55:45.305180Z 0 [ERROR]
[MY-010020] [Server] Data Dictionary initialization failed.
2020-05-07T07:55:45.305248Z 0 [ERROR] [MY-010119] [Server] Aborting
2020-05-07T07:55:45.305563Z 0 [System] [MY-010910] [Server]
/usr/sbin/mysqld: Shutdown complete (mysqld 8.0.20) MySQL Community
Server - GPL.
it says it cannot create a test file, but i have no idea why.
maybe unattended-upgrades broken rights.
the var/lib/mysql folder is owned by mysql user and group. so no idea why it couldnt make files in here.
thats really tricky.