It seems you did not click "CONTINUE" until the end of the upgrade.

Run the installation script in install/index.php. (Basically: do what the error message tells you to do).

I tried it: "http://www.........de/install/index.php" and press "Enter".
Unfortunately, I get an error message.

Do you need yesterday's error message by e-mail? I have saved it in a PDF.
The last lines were:
Copying "/ var / www / vhosts / .............. en / .............. en / skins_fallback_v5» ... OK.
Copying "/ var / www / vhosts // ............... en / ............... en / skins_fallback_v6» ... OK .
Then nothing happened.
".............." is my domain.

Do I now have to backup the backup-files from the "Backup" folder?
Must only the files be renewed or the database also?
How is the backup done correctly? Download, unzip, and then copy to the server?

I would like to apologize for these simple questions. I rather ask than to completely destroy something.

No you don't have to restore your backup for now; You should first do what I wrote in my previous messsage. If you can't because you get the message you screenshoted, rename the file ˋconf/maintenance.htmlˋ to ˋconf/imaintenance.htmlˋ.

http://b2evolution.net/man/maintenance-html

Also, yes, please send me your PDF.

Your advice has helped. Thank you very much. Unfortunately, there is only the "conf/umaintenance.html" file. I renamed it to "conf/imaintenance.html".
Then "install/index.php" was called. The installation ran normally through to the end. My hompage could only start normally, after the files was renamed/deleded .

I tried to find the error. Therefore, I installed the backup in a subdomain.Then I called the automatic-update again. (With own database). Now the update installation is aborted again. Then I waited some time and then updated the page with press "F5". That was the solution. The updated page has indicated that the update has not really aborted. I suspect that only the browser (Firefox, currently version) had a timeout. Is this possible?

No, I think your webhost kills the installer after some time. It would help if you turned on error display on your webhost and send me a screenshot of the error when the script stops and/or if you could send me your PDF.