Contributed By: Christopher Hill
I was having problems with clients displaying the following:

2003-03-18 08:37:54 08:37:54 Success IUCTL Starting
2003-03-18 08:37:55 08:37:55 Error IUCTL Library download error. Will retry. (Error 0x801901F7)
2003-03-18 08:37:56 08:37:56 Error IUCTL Library download error. Will retry. (Error 0x801901F7)
2003-03-18 08:37:56 08:37:56 Error IUCTL Library download error. Will retry. (Error 0x801901F7)
2003-03-18 08:37:56 08:37:56 Error IUCTL Failed to download iuident.cab from http://domaincontrol to C:\Program Files\WindowsUpdate\V4 (Error 0x801901F7)
2003-03-18 08:37:56 08:37:56 Success IUCTL Ignore above error, use local copy of iuident.cab from C:\Program Files\WindowsUpdate\V4

I managed to fix it by changing the server name defined in Group Policy to the FQDN of my SUS server (instead of http://domaincontrol I put http://domaincontrol.crgs.local)

Not sure why this wasn't working but it did fix the problem. I also set the FQDN in the SUS server options (Specify the name your clients use to locate this update server) just in case.