3.1.21 stopped my sites from working…. the new 3.1.22 seems to have fixed the problems (for me anyway).
I found this bug and patch in the wordpress bug tracker at http://trac.wordpress.org/ticket/3449
with the title
Disable RTE when editing comments
and the patch that does this at: http://trac.wordpress.org/changeset/4622
There is nothing in the bug/patch as to why this was done!
