On the two bugs

Which on pouring through the error logs I now think are one bug - caused by a mistake that I made in setting up the system. Tomorrow I will attempt a fix, which will involve taking the system down for a while. I will give a due warning when I need to do this. Cross all of your fingers and any other body parts you have to spare.
Bob

I think I've cracked it. The email sending system isn't working because I got a line wrong in configuration. That causes an error message in registration when the system tries to send a confirmatory email, but the registration works. It stops password changing working because the system sends an email resulting from that, and it stops reply working if the person you're replying to has selected 'send email on reply', because the system tries to send an email. Will attempt a fix tomorrow.
Bob.