Same problem.
From error logs:
PHP Fatal error: Uncaught Error: Class 'DOMDocument' not found in /var/www/html/wp-content/plugins/static-html-output-plugin/library/StaticHtmlOutput/HTMLProcessor.php:23
Stack trace:
#0 /var/www/html/wp-content/plugins/static-html-output-plugin/library/StaticHtmlOutput/SiteCrawler.php(341): HTMLProcessor->processHTML(Object(GuzzleHttp\\Psr7\\Stream), 'https://domain...')
#1 /var/www/html/wp-content/plugins/static-html-output-plugin/library/StaticHtmlOutput/SiteCrawler.php(258): SiteCrawler->loadFileForProcessing()
#2 /var/www/html/wp-content/plugins/static-html-output-plugin/library/StaticHtmlOutput/SiteCrawler.php(168): SiteCrawler->crawlABitMore()
#3 /var/www/html/wp-content/plugins/static-html-output-plugin/library/StaticHtmlOutput/SiteCrawler.php(44): SiteCrawler->crawl_site()
#4 /var/www/html/wp-content/plugins/static-html-output-plugin/library/StaticHtmlOutput/SiteCrawler.php(472): SiteCrawler->__construct()
#5 /var/www/html/wp-content/plugins/static-html-output-plugin/library/StaticHtmlOutput/Dispatcher.php(11): requir in /var/www/html/wp-content/plugins/static-html-output-plugin/library/StaticHtmlOutput/HTMLProcessor.php on line 23, referer: https://domain.com/wp-admin/admin.php?page=wp2static