Fatal Error: missed class in vendor/autoload.php when upgrading (no replies)

Hi,

during the upgrade to the latest Piwik version, I get this error:

Fatal error: Class 'ComposerAutoloaderInite8ad85ba0ba0c32158ef4e39d8e8f85f' not found in /var/www/web346/html/_piwik/vendor/autoload.php on line 7

In line 7 of the file autoload.php is:

return ComposerAutoloaderInite8ad85ba0ba0c32158ef4e39d8e8f85f::getLoader();

I re-installed Piwik new with the latest files from here [piwik.org], but this does not helped.
With this error I have no possibility, to log in, I have only access to my FTP.

Any idea, what this error could be and what I can do now?
Thank you.