Module in version of prestashop 1.6 uses "overrides". Overriding is a way to "override" class files and controller files. PrestaShop's ingenious class auto-loading function makes the "switch" to other files fairly simple.  PrestaShop allows to disable overrides, or these overrides are disabled automatically in some cases (like update of the shop) or while you apply security patch (presta from time to time releases security patches). If overrides will not be enabled - minimal order value by category module will not work.

How to enable overrides again?

To enable overrides in shop please follow this guide: disable / enable overrides in prestashop

Please note that this problem is applicable to prestashop 1.6 only. In prestashop 1.7 it is not required to care about overrides because module do not use them there.