World Wide Web Publishing Service error (Event ID 7023)

We set up Outlook Web Access on our Exchange Server 2003 machine
yesterday (running Windows 2003). SSL and form based login were
enabled, the certificate issued by an internal CA.

Everything worked fine all day. Overnight (at 8pm) about 20 minutes
after we checked Outlook Web Access from home. We got the following
error:

Event Type: Error
Event Source: W3SVC
Event Category: None
Event ID: 1036
Date: 21/09/2006
Time: 20:04:12
User: N/A
Computer: CHARLIE
Description:
A failure occurred while initializing the configuration manager for the
World Wide Web Publishing Service. The data field contains the error
number.
Data:
0000: 800706ba

Followed by;

Event Type: Error
Event Source: Service Control Manager
Event Category: None
Event ID: 7023
Date: 21/09/2006
Time: 20:04:14
User: N/A
Computer: CHARLIE
Description:
The World Wide Web Publishing Service service terminated with the
following error:
The RPC server is unavailable.

And the web page stopped loading (Page Cannot be Displayed). In the
morning we went to services, started the World Wide Web Publishing
Service and it is working fine (for an hour now).

We need out Outlook Web Access to be reliable, and since we dont know
why it stopped working we cant trust it at the moment. We are already
fighting a 'reliability battle' with some high profile users who will
insist on continuing to use their own POP3 accounts if Outlook Web
Access doesn't prove itself to them as a valid alternative.

Could anyone shed any light on the possible reason for these errors?
What is the RPC server?