After a recent upgrade to Movable Type, I found that attempting to comment using the comment forms on each entry page generated the error:

invalid Request: Corrupt Character Data For Character Set

The fix is simply to change the meta tag "content-type" on the Individual Entry template:

<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />

should become

<meta http-equiv="Content-Type" content="text/html;charset=utf-8" >

Then re-publish the individual entry archives.