Gerod points out in this blog post that if you install Windows 2003 SP1 onto your Exchange 2003 cluster nodes, you’ll need to install the hotfix in KB.841561.
If you don’t apply this hotfix, users will not be able to access OWA (they will receive 500 – internal server error), although Administrator accounts will be successful in accessing OWA.
If this scenario applies to you, double-check if the hotfix is available for download from the KB.841561 link — it’s not linked for download today — and if not, call PSS and request the KB.841561 hotfix. Requesting a specific hotfix from PSS is a no-charge call.
Updated 4/1/2005 (not an April fools prank): The hotfix download for KB.841561 is now linked from the KB article, or you can download it directly at this link. Note that this hotfix is for servers running Exchange 2003 SP1. If you can’t upgrade your server from Exchange 2003 RTM to Exchange 2003 SP1, you need to get the hotfix in KB.841560.