Mails not sent, not logged on server, just error

Hi,
after migrating to a new server, sending mails is no longer possible. ReCaptcha 3 is active, mail address is working. Added the SMTP-Plugin, but no chance: Mails are not sent. Just message:
“Beim Versuch, deine Mitteilung zu versenden, ist ein Fehler aufgetreten. Bitte versuche es sp?ter noch einmal.”
Contacted the provider, but there is absolutely nothing in their logs, even when checked live. Need help, as the customer depends on the reservations.
PHP: tried from 7.1.x to 7.4.x (actual)
No error report from WP.
Thanks so much for any hint,
Martin

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