Content not displayed of some html emails (Probably blocked?)

[brknkfr]

Steps to reproduce

1. Get an email with HTML content and open it.
2. Header is shown, content is blocked and not displayed.

Expected behavior

Content of email should be displayed.

Actual behavior

Content is not shown.

Mail app version

3.6.0

Mailserver or service

Dovecot

Operating system

Linux

PHP engine version

PHP 8.2

Web server

Apache (supported)

Database

MySQL

Additional info

In the network console of Firefox the request for the content of the email https://cloud.domain.tld/apps/mail/api/messages/1879/html is blocked with the error NS_DOMAIN_DOM_COEP_FAILED. If I open the requested URL directly in Firefox, then the content of the message is displayed. Funny though, when the preview content (text only?) is shown before actually opening the mail.

Possibly related to the errors in those bug reports: https://github.com/nextcloud/mail/issues/7550, https://github.com/nextcloud/mail/issues/7550