Craft 3 on new MAMP Pro 5 installation: "iconv(): Unknown error (22)" problem in /vendor/twig/twig/src/Extension/CoreExtension.php
Per my earlier comment, the convert_encoding filter is causing this, I think only in newer versions of Twig than what my live site running Craft 2 uses. 

I guess that probably somewhere in my site content, one of our content managers somehow inserted a character that convert_encoding is choking on. Rather than figure out where that is, and know that the site would blow up again the next time they did it, I've replaced bodyText|striptags|convert_encoding('UTF-8', 'HTML-ENTITIES')|slice(0, 200)|default('Read more') with
bodyText|striptags|slice(0, 200)|default('Read more')|raw, and while I have not tested this thoroughly to know that it works as intended in all cases, it's at least not blowing up an index listing of 100 articles because (I suspect) one entry has some weird character in it any longer.