[TYPO3-project-seminars] PHP Fatal error: Class 't3lib_refindex' not found

[Adrian Treusch v. Buttlar]

Hi List/Newsgroup,

when trying to complete/confirm a registration i end up with a blank 
page and the following error message in my log:
[30-Jul-2007 13:45:21] PHP Fatal error:  Class 't3lib_refindex' not 
found in E:\wwwroot_live\t3lib\class.t3lib_div.php on line 3995

system used: typo3 4.0.5, iis 6.0, php 5.2.1

any hints would be appreciated. i'd love to use this extension ;-)
adrian