Comparing with an example from a previous past at Securely creating fixlets to update OS user passwords , the only differences I see are that you are not declaring the top-level XML doc type, and not referencing the xml namespace attributes on the <BES> tag.
While I would expect that to behave the same between Parameter and SecureParameter, it may be worth trying to add those.
I’ll try to fire up my server later today to try it on my lab.
Also as a test, see whether you can target by ComputerID instead of name.