Problem installing SQL 2005 SP2 on a failover cluster

Last week I had some Upgrade problems on a failover cluster (SQL2000 to SQL2005). Well, the story hasn't finished yet. I still needed to install SP2….

Installation should be simple again. You just make sure you're running setup on the active node, select all options and go!

But no; halfway during the installation I get the message "A recently applied update, KB921896, failed to install". The setup continues and I can see the problem lies in upgrading the first (default) instance. Strangly enough, the second instance installs successfully.

Several google answers, but all just a bit different:

http://support.microsoft.com/kb/929151  

http://www.sql-server-performance.com/forum/topic.asp?TOPIC_ID=18116

http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1843298&SiteID=1

While I haven't figured out what the exact problem was, I have installed SP2 succesfully by failing over the instances to the passive node, and rerunning the setup on that node. Everything installed succesfully that time.