Hope this helps someone – took me a couple hours to figure it out. I commented on the princeblog.net post as well:
I had this same problem with IE.
I ran some diagnostics and inserted some print statements in the wp_insert_comment (comment.php).
Basically, my problem was the USER AGENT header; in my version of IE it was “Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; WOW64; Trident/4.0; EmbeddedWB 14.52 from: https://www.bsalsa.com/ EmbeddedWB 14.52; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR 3.5.30729; MS-RTC LM 8; OfficeLiveConnector.1.4; OfficeLivePatch.1.3; .NET CLR 3.0.30729)”
That’s 282 characters. Why’s that important? Because the wp_comments table has a comment_agent field with a length of only 255 characters.
The fix? Run this against your wp_comments table:
ALTER TABLE wp
.wp_comments
MODIFY COLUMN comment_agent
VARCHAR(512) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL;
Hope this helps!