Are all other extensions up to date?
Have you tried it with PHP 8.0?

In the Joomla backend, in the configuration under the SERVER TAB, switch “Report errors” to MAXIMUM
What error (screenshot) is there?
Which version of the NINA template are you using?
All questions that could help the developer solve your problem.

Thank you very much for your fast reply.
From my point of view, the extensions were up to date, except for the language extension "Germany" 4.1", for which the update was not accepted.
With PHP 8.0. it works. But PHP 8.0 will only be available until September 2024.
I downloaded the Nina template in December 2021. I can provide further information as soon as the backup of the site is restored (I hope this will be this afternoon).
I try everything, the problem must be solved, get back to you. Thanks.

Hi, the site has been restored, but it only works with PHP 8.0. or 7.4. Whenever I upgrade to 8.1. or 8.2. an error message appears. Screenshots are attached. On my other pages from Joomla51, an update for newer Joomla versions comes automatically, and these works with PHP 8.1. or 8.2. On these site, it says "joomla is up to date", although the site is still on Joomla version 4.0.2. I don't know what to do next. If it helps you, I can also send you password access via email at info@joomla51.com.
Thank you!

I am not a developer of J51.
just a try:
- Report bugs, change from maximum to standard.
- Also uninstall the German language... (then update?)
- Have you clicked on “repair” in the DATABASE in the Joomla backend?
- Have you really checked all extensions for PHP 8.1 readability?
- Was passiert beim umschaltne auf da sTemplate cassiopeia?

There was an error in Joomla here:
https://issues.joomla.org/tracker/joomla-cms/36171

It seems to be working again? (Link is ok) What was it?

To get rid of the "Deprecated" messages in PHP 8.1 or PHP 8.2, set the PHP variable display_errors to Off. How this can be achieved, depends on your hoster.

Nevertheless, you might want to inspect which extension causes these warnings. If you can do so, ask the developer of the extension for an updated version.

First of all, thank you very much for the various helpful feedback!

I tried out the inputs and was finally able to update the Joomla version to 4.4.1. The language file and the databases were not the cause of the problem. I went through all the activated extensions again and deactivated some of them, e.g. Breezing Forms, a contact form file, Recaptcha and some extensions that were unknown to me but not system-relevant. I also set the "Joomla update" setting to "Joomla! Next" in the configuration. And oh wonderfully the Joomla-Version update was available and could be implemented without problems. And finally, with this approch the PHP version 8.1. also works.

Thanks again, for the prompt support, you save my day. It is great in this forum when you realize that you're not left alone when you have problems.
Reto

Off-topic:

Gruezi, Reto, und viele Grüße in die Schweiz. Meine Frau und ich hoffen, auch bald wieder mal einige Zeit dort verbringen zu dürfen.

MfG, Rolf Dautrich