Vorschaubilder erzeugen dauert sehr lange

Are  you using the computer whilst the previews are being built?

The first step you should always take is to reset the Lightroom preferences.

https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/

It's recommended to backup your preferences before you reset the preferences to the default settings:

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

Unfortunately you haven't given us any information about your system. Therefore, all proposed solutions are a look into the crystal ball

Did this ever work? If yes, what has been changed on your system before the error occur?

Is your system up-to-date? Do you have installed all recent updates and patches for the operating system and the device drivers, especially the graphic driver. If you have a NVidia graphic card installed, make sure that you are using the recent Studio version of the driver, not the Game Ready version.

Please provide a copy of your 'System Info'. This can be get from the LrC Help > System Info menu item. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.