Michael,
Go to the person's forum/member picture, and click that..
The next screen is for their profile..
Now click "Private Message".
That 1st bar that says "Sent To [Username]" will already be pre-filled out for that member,
so don't add anything there.. just skip that bar, and move down to the next..
That's where you'll pop in your 'Subject'.
Then below that, in the big bog, you can type your message to them.
I think that maybe by adding their name *twice* in the member name bar,
maybe it was messing something up from the beginning..
Try this, and let me/us know..

Sam -