I just got this message trying to submit a comment to a Reader Blog:

Comment Submission Error
Your comment submission failed for the following reasons:

Invalid request
Return to the original entry.

I tried it twice, on a recently published blog with 3 comments already on it.. Then I tried to comment on one of my own older blogs and got the same message. Both comments I tried did not have links in them, and only one had code, for italics.  I was logged in while submitting each comment.

Update added as an edit: New problem: in attempting to comment on this post, I get the "timed out" pop-up message upon hitting "submit," which logs me out and leaves me without buttons under the comment box. Logging back in and returning fresh to the post with a box with buttons, the same thing happens, I am timed out again.

Update 12/31: I haven't had any trouble publishing comments so far this morning.

But see pseudocyants' report for adminstrators in his comment to this post on screwed-up urls he has discovered.

Second update 12/31:

Al Shaw: Thanks for reporting in with your comment on the thread.

I am not sure everything is fixed, proof is that I tried to post a reply to your comment many, many times to no avail over more than an hour, I got the timed-out pop-up over and over again, had to leave the thread, come back, and keep trying that way. I am giving up on publishing it and posting it here instead. I stay logged in, but cannot comment on this thread, I even tried to log out, that didn't work, it wouldn't let me. It says I am logged in, but then I get the pop-up message that my session is expired. Disconnecting from the internet and reconnecting didn't help. With previous comments this morning, I had no problem, but I sure had a problem trying to reply to yours. It's too much even for an addict like me--going elsewhere, will try the site again later, others may give up a lot quicker. I refuse to clear my cache just to use the site right now, would prefer to wait to see if things rectify themselves, as it will screw up too many other things for me.

A suggestion: consider making an announcement in the future on the main page when doing work? The fear of the unknown and all, rioting in the streets resulting, ya know. :-) Without an announcement, sometimes makes people think there is something wrong with their computer, and they spend a lot of time spinning wheels checking it out.

Third update, 12/31, 11:28am ET: Both commenting problems are still occuring. I just tried to publish a comment on Al Shaw's most recent "My Blog" post, in order to alert him to my updates because I couldn't post a comment on my own blog. On his blog, when submitting I got the Comment Submission Error/Invalid request  message. See y'all later after the gremlins have left.