The most likely cause of this error is that the 'version' key is missing from your configuration dictionary.
Steps to fix:
- Open <moodleinstalldir>/version.php
- Find the value in $version
- Open 'mdl_config' in database manager
- Insert key after last entry with name = 'version' and value = <value found in version.php>
If this doesn't solve your issue, visit http://moodle.org/mod/forum/view.php?id=28