During the upgrade of my Softgrid 4.1.1 server I encountered Error 1603.

The reason for this error is a missing installation path to the existing softgrid version. In the SoftGrid-server-setup.txt you'll notice the lines (the setup file is found in the users \Local Settings\Temp folder). Open the file with Trace32.exe (found the Configuration Manager 2007 Toolkit), this allows you to monitor all activity in the log file in realtime.

[2008-11-29 12:33:48] (3584:5960) INSTALLDIR: ``''.
[2008-11-29 12:33:49] (3584:424) Returning VersionString ``4.1.1.302 (RTM)'' for ProductCode ``{A4A45785-926B-4805-8180-58D330D419DE}''.

To fix the problem open registry, navigate to HKLM\Software\Microsoft\Windows\Currentversion\Installer\Userdata\S-1-5-18\Product GUID\InstallProperties (you can also search for the Returning Versionstring 4.1.1.303 (RTM). Edit the InstallLocation string value to your Softricity installation folder, in my case that was C:\Program Files\Softtricity\