Are you mentioning "HP" at the Product field? When you insert product and OS, each field automatically generates a tag. See if that resolve your problem.
(OFF) My opinion, they don't allow "HP" at the tags because: 1) It's quite obvious your product is HP; 2) The purpose of tag creation is to group customers with same issues/products/etc. Allowing "HP" at the tags, it'd multiply the number of tags and that's not interesting. Anyways I can be totally wrong, this is just my opion 🙂
Thanks for the reply - but no I didn't mention HP *anywhere* in my post, including the subject. I had to logout and login and then I was able to post the very same message without receiving a system error. Just a glitch, I suppose, albeit very frustrating.