Turns out this was caused by IIS. If you already have <action type="Rewrite" url="index.php" /> in your web.config, that should be enough with this addition to your index.php in the WordPress root installation directory

if (isset($_SERVER['UNENCODED_URL'])) { $_SERVER['REQUEST_URI'] = $_SERVER['UNENCODED_URL']; }

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【Windows IIS】IIS環境にWordpressいれたら思いの外いろいろアレだった話【Wordpress】 #WordPress – Qiita