Update not wokring

Hi, I have a Nextloud installation on a managed Webserver at the German hoster Hetzner. That means there is no access to the command line. Nextcloud was installed with the web installer. In the past I made several successfull updates by the web updater, but it doesn’t work anymore.

I have now installed 17.0.1 and want to update to 18.0.4. When I click on “Updater öffnen” (open updater) in the right upper corner the following message is displayed: Problem beim Laden der Seite, Seite wird in 5 Sekunden erneut geladen. (problem by loading the page, page will be reloaded in 5 seconds.)

I also tried the web installer in the folder of the installation, but it displays that it will not work in an existing nextcloud installation folder.

How can I update manually without command line?