Hi there,
For the first error while W3TC is active, please implement the fix mentioned here https://theme.co/apex/forum/t/the-preview-html-did-not-include-a-closing-html-message/14443
As for editing issue, would you mind updating your PHP to 5.6 or greater? It still in 5.4 which some features that the theme and other plugins may not perfectly work due to PHP version. Plus, it’s the recommended version.
And while I’m checking it, the site went down with this message
Fatal error: require_once(): Failed opening required '/home/p31595/domains/puppygroep.nl/public_html/testrobbin/wp-load.php
One of the WP core files are missing, are you currently updating or editing your site files? In that case, please let us know when you’re done.
And in the meantime, please try implementing this https://technumero.com/enable-keep-alive/. Your site’s Keep-Alive timeout is just 1 second, which may be, there is not enough time for the preview to render once it passed 1 second.
Thanks!