While attempting to debug a problem with a domain that uses SSL on a "private" IP address (https://www.virtualmin.com/node/37736) I temporarily disabled SSL support on that domain and reassigned it to the main shared IP address. This immediately broke my Postfix mail server, rendering it inaccessible from outside although the server appeared to still be running normally. Checking the networking configuration showed no obvious problem and disabling the firewall didn't help.
I speculate that the TLS certificate of the postfix server was overwritten due to some bug in this process, although I'm not sure about this because attempts to restore the original certificate (and private key) were unsuccessful, possibly due to a different bug. The problem is however repeatable - I eventually restored Postfix operation only to have it later break again the same way.