TYPO3 v6 has reached its end-of-life April 18th, 2017 and is not maintained by the community anymore. Looking for a stable version? Use the version switch on the top left.

There is no further ELTS support. It is strongly recommended updating your project.

Clear Caches and User Settings

You should always clear the cache tables. Go to "Clean up", scroll down to "Clear tables" and select the tables beginning with "cache_". Then press "Clear selected tables".

You might also consider clearing the Backend user preferences. This can avoid problems, if something in the upgrade requires this. Go to "Clean up", scroll to "Reset user preferences" and click "Reset backend user preferences".