Try turning off all your modifications via the Product Page. See if it works. Then turn on one at a time and each time see if it works. Or, go backwords and have them all on, see if it works (it doesn't), then turn off one at a time and see if it works.

Ahh, so I disabled every product but one, and it still wouldn't let me post replies. The thing is, that one product I did not disable actually does not allow me to. I try setting it to Disable, then pressing "Go", but it does nothing. Even with the Uninstall option. Do you know if there is a way to force the modification out of the database?

If you are having problems with that one, you should ask in the modification thread for help disabling it.