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There is a pretty fundamental error that needs to be fixed here. webmentions_open is called without an argument when one is mandatory in the latest version
PHP Fatal error: Uncaught ArgumentCountError: Too few arguments to function webmentions_open(), 0 passed in /***/wp-content/plugins/webmention/includes/class-discovery.php on line 126 and exactly 1 expected in /***/wp-content/plugins/webmention/includes/functions.php:535