[SOLVED] Upgrade 2.0 -> 2.1, problem with flow commands

I am getting following error while trying to proceed with upgrade 2.0 -> 2.1 steps.
https://www.neos.io/download-and-extend/upgrade-instructions-2-0-2-1.html

./flow flow:cache:flush --force

PHP Fatal error: Uncaught exception ‘TYPO3\Flow\Package\Exception\InvalidPackageStateException’ with message ‘Package “Doctrine.Instantiator” is already registered as “doctrine.instantiator”.’ in /var/www/…/Packages/Framework/TYPO3.Flow/Classes/TYPO3/Flow/Package/PackageManager.php:730
Stack trace:
#0 /var/www/…/Packages/Framework/TYPO3.Flow/Classes/TYPO3/Flow/Package/PackageManager.php(1067): TYPO3\Flow\Package\PackageManager->registerPackage(Object(TYPO3\Flow\Package\Package), false)
#1 /var/www/…/Packages/Framework/TYPO3.Flow/Classes/TYPO3/Flow/Package/PackageManager.php(887): TYPO3\Flow\Package\PackageManager->registerPackagesFromConfiguration()
#2 /var/www/…/Packages/Framework/TYPO3.Flow/Classes/TYPO3/Flow/Package/PackageManager.php(822): TYPO3\Flow\Package\PackageManager->scanAvailablePackages()
#3 /var/www/…/Pack in /var/www/…/Packages/Framework/TYPO3.Flow/Classes/TYPO3/Flow/Package/PackageManager.php on line 730

Remove Configuration/PackageStates.php from the application root folder. This changelog explains what is happening https://flowframework.readthedocs.io/en/3.0/TheDefinitiveGuide/PartV/ChangeLogs/300.html#task-increase-packagestates-version

This is also the changelog for the Flow version that 2.0 uses

Thank you, that works.