Various loyal readers, and one loyal co-blogger, have had problems in using the comments form when using Internet Explorer. In one case, this was solved by using the option to comment anonymously while filling in his own name; in another case, only flipping to FireFox or Chrome worked.
As Blogger handles all of the technical stuff automatically, there's not much I can do; I've tried "turning it off and turning it on again" by disabling comments then re-enabling them. If that doesn't do it, all I can suggest is trying FireFox or Chrome. My apologies.