How strangely coincidental....
I have been doing some work with Web Service Consumers this morning and had the same sort of error when attempting to invoke the service. This may or may not help you (its a bit of a long shot) but my issue was concerning the account credentials I was attempting to use. Basically the ACL allowed access to the user via the notes id but not the web id. As soon as I added the common name to the ACL the error was resolved.
Hope it helps