I know that comments don't work, because I intentionally broke them. I've temporarily disabled comment posting across the site. I'll emphasise that: comment posting will now no longer work.
At least until I turn it back on.
What happened is that I upgraded the site to MT3 tonight. No problems there. However, I ran into trouble upgrading the MT-Blacklist module to the MT3-compatible version 2 -- my webserver's missing a certain critical Perl module, or so I suspect, given that all I have to go on is a collection of information-free Internal Server Error pages.
MT-Blacklist was invaluable to me, as it provided a defence against those thick spammers insistent on running repeatedly into my blacklist, just like the slow kid at school used to run head-first into brick walls. So, with no anti-spam measures in place, I thought it would make my life easier just to disable comments for a while.
A little more fiddling to verify if I'm right, and then a polite email sent in the direction of my hosting provider will hopefully do the trick.