Please delete CMB2 plugin, deactivate Under Construction and enable the Maintenance mode in the Theme Settings > Maintenance mode, then create the page or chose the page you want to show as the maintenance page and choose the template “Maintenance” as shown: https://gyazo.com/d4d6aaa7faca0afc287fe530fe7bbff3
Please enter each widget where you inserted HTML tags and make sure the code is correct and all the tags closed.
Please deactivate all the plugins except these ones:
WPBakery Page Builder or Elementor
Contact Form 7
MailChimp for WordPress
If the problem has gone, activate the plugins one by one, checking the issue to detect which one causes the problem.
In case, the problem remains, even after all the plugins are deactivated, provide your site admin access (insert the site credentials into the Private content block under the message area) and confirm the permission for plugins deactivation, switching to the parent or default theme. This testing may take up to 15 minutes. You would better make the full backup of your site.