Usermin Webmail not sending


I don’t often have occasion to use webmail on our virtualmin servers but, needed to this morning. Mail was displayed correctly and I can read it as expected but, when I try to reply to a message, it just times out without an error that I can see. Same issue on two servers, both running usermin 17.41 and Authentic Theme 19.19 but one server is Ubuntu 14.04.2, the other is Ubuntu 16.04.1. Both using postfix/dovecot as configured by virtualmin.

Nothing recorded in the mail logs and, I can’t find anywhere usermin might be logging the attempt. What should I look at?