Hi, i finally succeeded in upgrading to NC13 without any major headache:
I installed NC13 into a separate folder, copied the “old” config.php to the “new” config directory.
the config.php contains the data directory outside the web- root folder.
I accessed nextcloud13/index.php and after upgrading i entered my credentials and everything was fine again - without fiddling around with the “old” NC 12 installation.
Thanks and best regards, Hitam