The information "it runs the script bin/updatedb.sh" was useful in a way. I uploaded all files with an FTP/SFTP client from my Windows computer. As every Linux developer should know, Windows doesn't have that rwx-style attributes. So all uploaded files don't have the x attribute set by default. This will prevent script files from being executed on Linux.
I manually set the x attribute for all files in bin/ (with the upload client) and then the database schema update worked just fine.
This information should have gone in the initial error message, instead of a halfhearted "update failed" or what it was. Or at least in the upgrade/upload instructions. Does that feedback help any user with the same problem, or the developers to give mor useful error messages?