"Can't find file mysql/plugin.frm" when starting up MySQL
If you have mysql data directory at non-standard location and your system has apparmor, ensure that /etc/apparmor.d/usr.sbin.mysqld has your data directory whitelisted. The configuration should look like:

/mysql/data/dir/ r, #trailing slash is mandatory
/mysql/data/dir/** rwk,


Apparmor daemon has to be restarted after you make any changes to config.