MyBB Community Forums

I uploaded the files for the 1.6.4 upgrade, upgrading my 1.6.3

I go to www.mydomain.com/install/upgrade.php

And followed the upgrade setup

Then it said upgrade successful and the install folder has been locked but when I visit my forum it shows an error

MyBB has experienced an internal error and cannot continue.

Error Type:
MyBB Error (42)
Error Message:
Your board has not yet been upgraded. Please do so before attempting to browse it.

let us know which files you uploaded for upgrading .. latest 1.6.4 package from downloads page needs to be
uploaded by overwriting earlier installed files (keep copy of config.php & settings.php AND database backup)

I have a database backup config and settings backed up. I uploaded the files by overwriting.

My problem is even though it says that upgrade was successful, after surfing my forum for a minute or maybe half a minute, it will show an error about the forum not upgraded yet

forum URL please ...

So you ran through the upgrade script fully?? It can't work fine for a time and then suddenly do this by itself.

Well it works now, what happened? Is that a bug?