That's strange. I don't think this is a CSE HTML Validator message. I cannot find that message in CSE's code. A couple guesses:
1. Perhaps related to anti-malware software. You may want to try turning it off. 2. A bug/problem with Opera opening HTML documents. You could try switching your default browser to IE or Firefox and see if this message still appears.
If you discover the cause, then please let me know.
I flipped over to Firefox and the problem went away. This makes sense. I think the problem started when I flipped from Opera 10.1 to 10.5. I have reported this to the Opera people. Judging by the screaming, 10.5 is a lemon.
It turns out the problem was with the Opera 10.5 installer. When it sets up the association, it does the DDE stuff properly for when Opera is already running but leaves off the "%1" when opera.exe is launched. Changing the default to Firefox then back to Opera fixed it.