NC 12 -> 13 upgrade fails - Composer autoloader not found


after the upgrade without errors via web interface, there is only the message:

Composer autoloader not found, unable to continue. Check the folder “3rdparty”. Running “git submodule update --init” will initialize the git submodule that handles the subfolder “3rdparty”

How to fix this?

There is no subfolder 3rdparty at /var/www/nextcloud/


I restored nextcloud and upgraded manually by replacing the nc folder. After restoring the config file, the update finished without errors.
5743314a71e972ae46a14b36b37394d4545915aa5f32d9e12ba786d04c1f1d11 nextcloud-13.0.1.tar.bz2