Not sending mails since update to 5.4

Where can we see the website in question?

Hi, thanks for getting back.

Please see here: https://alt.s-e-p-a-g-o.de/unternehmen/kontakt/
(remove “-” from Domain)

This is an old installation. Mail sending worked fine with WP prior 5.7 and CF7 prior 5.4.
– I then updated CF7 to 5.4 and it still worked(!)
– Then, after updating WP to 5.7, it is failing…
No caching, no honeypot, no antispambee, no smtp.

Same error from JSON:

{
	"into": "#wpcf7-f4-p21-o1",
	"status": "mail_failed",
	"message": "Es gab einen Fehler beim Versand. Bitte versuchen Sie es erneut.",
	"posted_data_hash": "40538ac772d68d205ff15eac56a8dec5",
	"errorInfo": "Could not instantiate mail function."
}

Btw:
flamingo is saving the unsend mails with default values:
Subject: [your-subject]
To: [your-name] <[your-email]>
Don’t know if this bug is related to original issue. (no real send-to value?)

What other plugins and theme do you use on the site?

I got it!
It has something to do with this ticket:
https://core.trac.www.ads-software.com/ticket/52822

Long story short:
Updated PHPMailer is faulty on some server configurations.

For now i changed the content of /wp-includes/PHPMailer with the newest files from here: https://github.com/PHPMailer/PHPMailer/archive/refs/tags/v6.4.0.zip

That did it for me.
Thanks for your time.

@takayukister

A lot, but for e-mail: WP-Mail-SMTP.
And i use the extension: Contact Form 7 Extension For Mailchimp.

I did try do deactivate this plugins but that did not solve the problem.

My problem is solved. The problem was the comibination with the Autoptimize plugin.
https://www.ads-software.com/support/topic/contact-form-7-major-update-breaking-js-optimization/

Excluding wp-includes/js in Autoptimize solved the problem for me.