HTML mails not displayed

[pippcat]

Steps to reproduce

1. Open mail app in Nextcloud
2. Open an HTML mail in mail app
3. See that the content isn't displayed

Expected behavior

HTML mails should be displayed properly.

Actual behavior

HTML mails aren't loaded

Mail app version

3.7.24

Nextcloud version

29.0.15

Mailserver or service

No response

Operating system

No response

PHP engine version

PHP 8.1

Nextcloud memory caching

local: \OC\Memcache\APCu distributed:\OC\Memcache\Redis

Web server

Nginx

Database

MySQL

Additional info

Opening plaintext mails in the Nextcloud Mail app works like a charm. HTML mails aren't displayed though. Here's an example image:

It's independent of the HTML content being loaded from remote or included in the mail itself.

I had a look in the Nextcloud protocols but couldn't find anything useful. The browser console shows this entry only for not displayed HTML mails:

[iFrameSizer][Host page: iFrameResizer0] IFrame has not responded within 5 seconds. Check iFrameResizer.contentWindow.js has been loaded in iFrame. This message can be ignored if everything is working, or you can set the warningTimeout option to a higher value or zero to suppress this warning. 

Any idea what could be the issue for this behavior or which other information to collect figure out what's going on?

[ChristophWurst]

Could you please open the browser console and try again? Are there console errors or HTTP requests with 5xx statuses?

[ChristophWurst]

3.7.24

FYI you seem to be running Nextcloud 29 or older. Those have reached EOL. Please plan an update to 30 or later.