Thanks for posting the text of the error. That helped me narrow my search. From my research, it seems this is an issue with cookies. I've made some changes so please rest and let me know if you still get the same error.
Now i'm getting an error for the first time.
I used the "mark as read" button since forever with no problems at all, now this appeared:
An Error Has Occurred!
Unable to verify referring url. Please go back and try again.
I use IE 8.0.6001.18702 (I know , i know, IE is not the best) on Windows XP 32-bit (i know, i know, i'm old-fashioned)