Xhr.js:220 - Leading to tons of 404 (Not Found)

Hi, I freshly migrated my data to NextCloud (fresh self-hosted installation).
NC 26 Groupware, Ubuntu 22.04 LTS, PHP 8.2., Cyrus IMAP

The system is quite slow and when looking at the error messages in the Browser developer console, I see tons of these messages when opening an e-mail in the Mail app (here a newsletter example):

xhr.js:220 GET https://<my_nextcloud_url>/index.php/apps/mail/api/avatars/url/noreply%40newsletter.heise.de 404 (Not Found)

In the user profiles, I disabled the use of updating Avatars from Social Media Accounts. Do you have any pointers what could be going wrong?