Recently, while attempting to build a Japanese MSI using WiX v3.0, I received an error message that looked like the following:

light.exe : error LGHT0217 : An unexpected external UI message was received: ????????????????????????????????????????????????????????????? ???? 2738 ???

I wanted to see the full text of this error message, so I temporarily changed the product language setting in my WXS file from 1041 (the LCID for Japanese) to 1033 (the LCID for English) and re-ran the build.  When I did that, the error message was more readable:

light.exe : error LGHT0217 : An unexpected external UI message was received: The installer has encountered an unexpected error installing this package. This may indicate a problem with this package. The error code is 2738.

This message made more sense to me, so from there, I looked at the Windows Installer Error Messages reference for error code 2738 and found the following:

Could not access VBScript run time for custom action [2].

While attempting to narrow down this error further, I found a couple of useful blog posts.  As described in this blog post by Heath Stewart, error code 2738 can occur on Windows Vista systems (like the one I was using in the above scenario) if an MSI contains a script-based custom action and the VBScript runtime is registered in the HKEY_CURRENT_USER hive instead of or in addition to the HKEY_LOCAL_MACHINE hive.  Also, as described in this blog post by Bob Arnson, four of the Windows Installer Internal Consistency Evaluators (ICEs) are implemented in VBScript.  The offending ICEs are ICE08, ICE09, ICE32 and ICE61.

In my build output, I noticed that light.exe had reported some warnings for ICE03 before displaying this 2738 error.  From this, I made an educated guess that it was probably failing while running one of these four ICEs.  Then, I used some of the advice that I listed in one of my previous blog poststo unregister vbscript.dll and jscript.dll from HKEY_CURRENT_USER to resolve this error:

1. Click on the Start menu, choose Run, type cmd and click OK
2. To unregister the VBScript engine, run this command:  reg delete "HKCU\SOFTWARE\Classes\CLSID\{B54F3741-5B07-11CF-A4B0-00AA004A55E8}" /f
3. To unregister the VBScript engine on a 64-bit OS, run this command:  reg delete "HKCU\SOFTWARE\Classes\Wow6432Node\CLSID\{B54F3741-5B07-11CF-A4B0-00AA004A55E8}" /f
4. To unregister the JScript engine, run this command: reg delete "HKCU\SOFTWARE\Classes\CLSID\{F414C260-6AC0-11CF-B6D1-00AA00BBBB58}" /f
5. To unregister the JScript engine on a 64-bit OS, run this command: reg delete "HKCU\SOFTWARE\Classes\Wow6432Node\CLSID\{F414C260-6AC0-11CF-B6D1-00AA00BBBB58}" /f

Once I did that, my build started working as expected.  I still do not know how this information ended up in HKEY_CURRENT_USER on my system, particularly because the same build worked correctly on the same machine a couple of days ago.  However, I am now back into a state where I can build successfully, and I know of a possible workaround to try if I end up with similar build issues in the future.

<update date="12/19/2011"> Added steps to unregister the VBScript and JScript engines on 64-bit OS's. </update>