unable to change secure_file_priv in my.cnf
I do not know in Linux but in Windows 10/MySQL80 (my.ini instead of my.cnf) the option is spelled secure-file-priv  and not secure_file_priv. Do not forget to restart the service once the change is made.

I sadly lost one evening before finding my error. This is especially confusing as the SQL query to check the variable value is SELECT @@secure_file_priv.