@donq said in Opera and Fritzbox 7530 MP:

Expired certificate certainly needs to be replaced

But what is the best way to obtain a valid SSL certificate,
Windows Update, or how should I proceed ?

Problem is not in Windows, but in Fritzbox. Most likely Fritzbox certificate is either self-signed or replaced by some Letsencrypt one (which is expired)(I read this from Fritzbox help pages) and you should either trust its certificate (for windows) or renew it. Both are non-trivial tasks for people, not familiar with certificates; as I have no Fritzbox here then I can't guide you step by step either

But as other people said, it is usually easier to make browser (Opera) trust such certificates and not fiddle with windows or Fritzbox certificates.

About suggested opera://flags/#unsafely-treat-insecure-origin-as-secure - just copy-paste that text into address field and press enter, you will see something as next, set it to enabled and fill it with your fritzbox address (it may be http://fritz.box too - you need to try both):

Restart opera and try to connect - maybe it works this way.

The Internet browser flags connections to websites that are not encrypted or only partially encrypted as unsafe, as data entered on such websites could be stolen by third parties. However, since you are accessing the user interface of your FRITZ!Box within your home network and no data is being sent over the Internet, you can ignore the message.

Click on a blank spot on the page and type thisisunsafe.
First of all, thank you for your feedback.

The “thisisunsafe” command worked,
but I'm not sure if I'm really on the safe side.

Or, enable the "Insecure origins treated as secure" flag at the URL opera://flags/#unsafely-treat-insecure-origin-as-secure and add the site to the flag's list. You might have to restart Opera afterwards.

This command also works, but I have absolutely no idea
where this command should be entered in the Windows security settings, or anywhere else for that matter.

Explanation:
When you reach a certain age, in this case 70, it becomes difficult to keep up with the digital world and to implement help correctly.
Also, my English isn't really good enough to keep up in English-language forums, so I have to translate everything with “deepl.com.”
This is very tiring and time-consuming for me.

Thank you @all for your helpfulness and patience.

Klaus

Or, enable the "Insecure origins treated as secure" flag at the URL opera://flags/#unsafely-treat-insecure-origin-as-secure and add the site to the flag's list. You might have to restart Opera afterwards.

If your OS and your browsers are up to date, you shouldn't have problems with certificates.

Expired certificate certainly needs to be replaced

But what is the best way to obtain a valid SSL certificate,
Windows Update, or how should I proceed ?

After further research, I now believe that in my case, it is due to an expired SSL certificate.

Certificate problem was my initial guess too, just as there can be different security warnings, I wanted more information.
Expired certificate certainly needs to be replaced

The page I select, in this case the user interface of the FritzBox 7530 MP, is not secure.
This is also the case with other browsers (Firefox, Edge, Chrome, and Internet Explorer).
Similarly, dial-up via notebook and mobile devices does not work.
After further research, I now believe that in my case, it is due to an expired SSL certificate.
However, this is only my layman's opinion!
I have taken some screenshots and attached them here.

Translated with DeepL.com (free version)!

Opera_fritz.box_2_with_https.jpg

I always receive a security warning.

What kind of warning? Do Edge and/or other browsers work without problems?