Upgrade OC 10.0.2 -> NC 12.0.1

When trying to migrate from OC to NC I got the following error:

Current version is 10.0.2.
Could not determine migration path to Nextcloud.

I downloaded the index.php from https://download.nextcloud.com/server/installer/migrator/index.php2 to $owncloud_dir/updater/ and started the migration tool via OC-Web-GUI

isn’t the migrationtool updated yet ?

If you download the code manually, the upgradre from OC 10 -> NC 12 should be possible.

But shouldn’t the migration file be usable ?
how do I do it manually then ? I got owncloud 10.0.2

Take a look at the last section here.