On a board with APCu installed and configured, the installer script appears to be confused because the cache directory contents do not match what it is expecting.

On the pre-update check using the advance update method, it gives the error message: "The installer has detected an issue with a cached file. Try reloading the page and/or manually clearing the cache to resolve the issue. If you require further assistance, please visit the phpBB support forums."

Also at the end of the update process, the installer appears not to be clearing the APCu cache and it must be cleared manually in the ACP before the forum will behave normally.

As a work-around, the APCu may be temporarily disabled in config.php during the update process.

This issue has been noticed on several previous phpBB updates.