The KB article you referenced does indeed indicate there is a known issue at FP8 - however I am not having any problem here, in fact I just deleted a user with this same agent.
My server is at 9.0.1FP8IF3, my client at FP8.
My statement is slightly different from the kb article, instead of calling the method, I'm assigning the note ID of the adminp document:
noteID$ = adminp.DeleteUser( FullName.Canonical , False , DelMailFile , "Terminations" , False )
If you're using 'Call' (as noted in the KB article) you might try rewriting the agent to grab the noteID$ as I'm doing to see if that will work for you.
Oh, another difference - In my agent I'm using an actual integer value (of 2) instead of the constant 'MAILFILE_DELETE_ALL'. you might try that as well.