Can't get iso-8859-1 to work for 'umlauts' (Debian / Plesk)

I think I have configured ISO-8859-1 at all necessary locations, but still, my pages are displaying black '?' instead of German umlauts! My server runs Plesk 17.8.11 / Debian 8 / Apache 2 / PHP 7.1 (FPM). Plesk > Websites & Domains > mysite.com > PHP Settings: Additional configuration directives: default_charset = "iso-8859-1" Plesk > Websites & Domains > mysite.com > Apache & nginx Settings: Additional directives for HTTP: AddDefaultCharset ISO-8859-1 Additional directives for HTTPS: AddDefaultCharset ISO-8859-1 Additional nginx directives: charset iso-8859-1; /etc/apache2/apache2.conf: AddDefaultCharset iso-8859-1 /etc/php5/fpm/php.ini: default_charset = "iso-8859-1" /etc/php5/apache2/php.ini: default_charset = "iso-8859-1" .htaccess in the root directory of the affected domain contains: AddDefaultCharset ISO-8859-1 I hope it is okay to reference one of the affected URLs here to allow checking the source and encoding of the page: https://www.psychotherapiepraxis.at/gute_therapie.phtml All page content is saved in .html files, so the pages' html code isn't retrieved from a database. Any idea what might be the culprit?