pichichi

It sounds weird, but routers can cache DNS. If you have flushed the DNS on your devices (not the same as clearing browser cache and cookies) and they all use the same network, try resetting the router.

1. Clear caches on plugins such as wpo-minify you have installed
2. Check for other caches, sometimes themes have built in caches
3. If you use a CDN like Cloudflare, clear that too
4. Check the site now works in your browsers private mode, if not
5. Make sure all the required PHP extensions are installed, sometimes you need to add them manually after updating. There is a list here: https://make.wordpress.org/hosting/handbook/server-environment/#php-extensions
6. Ask your host if you have a PHP object cache and ask to clear

Check you have all the required php extensions re-enabled, not all extensions are enabled automatically when switching versions: https://make.wordpress.org/hosting/handbook/server-environment/#php-extensions

Hey, try asking here, they are more likely to be able to help: https://wordpress.org/support/plugin/elementor/

Settings>General>Membership>Anyone can register

Make sure this is unchecked or else anyone can register via your login page.

What does your hosting error log say? The exact urls that are giving the 4xx errors should show up there, as well as any other errors.

Also, does “site health” on the WordPress dashboard say any PHP modules are missing? Sometimes changing PHP version does not install all the modules you would need.

We can split this into two things to solve 1) remove the via part after the email address and 2) change the email.

Part 1

To get the “via webhost” part removed, you need update some records at your domain name host to prove your website host is authorised to send mail. If you don’t do this mail will probably also end up in spam. Speak to your web host or look up your hosting’s documentation about setting up (a) SPF record and also see if they support (b) DKIM

Part 2

To change the notificatiuon email From address, search for a plugin that has good reviews and is well supported, or if you are using a child theme, add the follwing snippet to your functions.php, changing the email address in bold to your chosen email. Don’t delete the quote marks.

// Change notification sender address
function change_notification_email( $wp_mail_from ) {
    return 'yourNewEmail@deklokborgloon.be';
}
add_filter( 'wp_mail_from', 'change_notification_email' );

but what’s the point 

If this is dues to a plugin like Threadi says, some valid reasons for encoding data passed through a plugin would be to prevent malicious code like cross site attacks, and like Threadi says, best to ask the plugin developer their reason, only they will know