TYPO3 v9 has reached its end-of-life September 30th, 2021 and is not maintained by the community anymore. Looking for a stable version? Use the version switch on the top left.
You can order Extended Long Term Support (ELTS) here: TYPO3 ELTS.
You should also clear all caches when upgrading.
Go to "Maintenance" and click "Flush":
Additionally, after an upgrade to a new major version, you should also delete
the other temporary files, which TYPO3 saves in
typo3temp/. In the
Install Tool go to "Clean up" to do so.