following the first answers, this is the solution for linux (ubuntu 18.04) users. You can jump right to the solution a little lower...
I took like 4 information sources to get it all going for me, thats why I put it all together a little more detailed...
After preconfiguring fluxCP (see github wiki) I had the same problem mentioned here after hitting fluxCP "update all".
To make sure its actually this error, you need to enable full error log in the error.php:
sudo nano /var/www/html/config/error.php
now set this from false to true:
exit with STRG+X and confirm save with Y and RETURN
Then you are able to see the full error log:
My working Solution
open the mysqld.cnf with write permissions (e.g. sudo):
sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf
append this in the [mysql] section:
sql_mode=NO_ENGINE_SUBSTITUTION
exit with STRG+X and confirm save with Y and RETURN
now restart the mysql service:
sudo systemctl restart mysql.service
