KMail - HTML view often gets lost after some time

Hallo Discuss Community,

since some months I am experienced, that I cannot see the HTML view in KMail messages, if the message are opened some times after received. If it has opened in few minutes as received, you can see it in HTML format. Here an example:

Here can you see on the right bar “keine HTML Nachricht” (trls.: no HTML message), but it is not true! I have already red this message in HTML form.

Why does it happen?
There is a way to solve this issue?

I hope I’ll get some help here …

My KMail settings concerning HTML:

Composer → Format → Improve plain text version of HTML message
Security → HTML messages → Prefer HTML to plain text: NOT ACTIVE
Security → HTML messages → Allow messages to load external references from Internet: ACTIVE

Information about the system

openSUSE Leap 15.6
KDE-Plasma 5.27.11
KDE-Frameworks 5.115.0
Qt-Version 5.15.12
Graphic platform: X11
Kontact: 5.24.5