Craft CMS Control Panel/admin 500 error with matrix, neo or supertable?

Chances are that you might need to adjust a php setting

Plugins and features like supertable, matrix and neo generate a huge number of form fields.
PHP by default limits the max number of fields to 1000.
Chances are that you might want to increase the number of fields.

; in php.ini

You can only set this in php.ini.
With fortrabbit, you can modify this value in your php control panel.