800a01b6 Create Mailbox Asp

I've created an ASP web page where users in our organization can create
Active Directory computer accounts.

The web page is running on a Server 2003 SP1 IIS 6 installation. The
Exchange System Manager is running on the web server and Exchange SP2 has
been installed.

The web page uses ADSI code to create a user, and then CDO code to create
the mailbox e.g.

set oOU=GetObject(<LDAP string for OU>)
set oUser=oOU.CreateUser("user",<CN for user>)
' set a bunch of attributes
' now create mailbox
set oMailbox=oUser
oMailbox.CreateMailbox <string representing MTA>

Now, when I connect to this site using my domain admin credentials, it
works. However, if a user connects to this site, the user account is
successfully created in AD but upon reaching the "CreateMailbox" line, I get

Microsoft VBScript runtime error '800a01b6'
Object doesn't support this property or method

Thinking that this was most likely a permissions issue, I took the .asp page
containing the code and converted it to a .vbs file. Then, I logged in as the
user account and used cscript to execute the .vbs file. Both the user account
and the mailbox were created succesfully.

I don't understand why the code runs under the context of a user account as
a vbs script, but doesn't run as an ASP page unless the user is a domain
admin. It's not permissions on AD or Exchange or the code wouldn't have run
as a vbs script. What is different about running it as an ASP page?