Solved After the installation of Kunena 6.2.1 to J4.4. no searching and expanding works

This is easy to determine. Go to System -> Extensions and search for this extension. Disable everything that belongs to this extension.
Basically: Always use the Joomla and Kunena default template during troubleshooting.
Disable Add Suffix to URL in the Joomla Global Configuration. This can leads to errors.
Furthermore: Why have you activated reCaptcha if your users are created manually? If the registration is disabled, no spambot can register. Disable reCaptcha.

Please create a configuration report again afterwards.

Disabling the Nicepage will lead to the fact, that contents generated in the Nicepage will disappear as well as the template, that was created by this extension and brought within the editor.

I do not have an enabled captcha (Extensions Manage). The only thing related to the captcha is publicly visible content created with Nicepage, where the contact form, wherein the creation of the Joomla Theme was added secret and public keys, not to flood the address used as a recipient of the contact form message.

Also finding the Extension in the Extension Manager did not bring anything related to Nicepage. The Nicepage was renamed to SVJKrskova783-784, and even that I disabled that, it still returns to 403, so that leads me that it does not have anything to do with Nicepage.

I installed the Kunena board to my localhost and installed there demo-content. I adjusted the privileges and scheme of the sections and Categories, and it seems to be working fine now.

Anyway, thanks a lot for your patience and assistance rich.

Thanks again,
M007.file