Emoji mb_detect_encoding() error when updating to latest Craft 2.6.x
Solved it. The problem was with this field:

&lt;input type="hidden" name="fields[gdprPermissionsList][permissionSales][fields][lastChangedDate][date]" value="{{ now|date("m/d/Y") }}"&gt;


The [date] part was to much and it saved the value in Craft as a literal string: {"date":"07\/11\/2018"} instead of the formatted date.

In one of the Craft updates that throws an error, maybe because it ses it as an object/array due to the curly brackets or maybe because that string has an emoji shortcut in it?