Server script triggered via XML = error 200, even though logged in a [Full Access]?

I ran into an odd problem.  I was triggering a script to run on the server via the XML interface (yes, the old one).  I was passing credentialls for a [Full Access] account.  The script was pretty simple, just going to a layout, create a record, set a field.  However, it was giving me an error:

...XML 200 1071 Web Scripting Error: 200, File: "AFile", Script: "Handler", Script Step: "New Record/Request"...

I was able to fix this error by turning on the 'run with full access' on the script itself.  But I was already logged in as full access, so why did I have to do this extra step?

(Side note:  this script worked just fine if I made the XML call directly from a web browser instead of from a web viewer inside a DB hosted by the same server.)

Thanks,

Justin