I have been getting this for weeks when trying to close a question as duplicate. This happens on both OS X with Safari and on my iPad (3rd gen) in Safari. I also tried Chrome, same issue. I have been unable to close questions as duplicate for weeks.
Is there any way to diagnose/resolve this on my end, is it perhaps tied to my user account somehow? Or is this a (known) bug?
UPDATE:
Recently I see this code directly above the red error box after entering some text in the text field:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> </html>