Since WordPress 5.2 there is a built-in feature that detects when a plugin or theme causes a fatal error on your site, and notifies you with this automated email.
In this case, WordPress caught an error with one of your plugins, Ninja Forms.
First, visit your website (https://carpnature.nl/) and check for any visible issues. Next, visit the page where the error was caught (https://carpnature.nl/wp-admin/admin.php?page=wpdevart-calendars) and check for any visible issues.
Please contact your host for assistance with investigating this issue further.
If your site appears broken and you can’t access your dashboard normally, WordPress now has a special “recovery mode”. This lets you safely login to your dashboard and investigate further.
To keep your site safe, this link will expire in 1 day. Don’t worry about that, though: a new link will be emailed to you if the error occurs again after it expires.
When seeking help with this issue, you may be asked for some of the following information:
WordPress version 5.6.2
Current theme: X – Child Theme (version 1.0.0)
Current plugin: Ninja Forms (version 3.5.1)
PHP version 7.4.5
An error of type E_COMPILE_ERROR was caused in line 788 of the file /home/zomer2010/domains/carpnature.nl/public_html/wp-content/plugins/ninja-forms/ninja-forms.php. Error message: require_once(): Failed opening required ‘/home/zomer2010/domains/carpnature.nl/public_html/wp-content/plugins/ninja-forms/includes/Fields/CreditCardCVC.php’ (include_path=’.:/opt/alt/php74/usr/share/pear’)