Okay… So far, the only thing I’ve found that changed was that the upgrade of the roundcube package replaced Hestia’s /etc/roundcube/config.inc.php with the default that comes in the package. Somehow, however, it renamed the one that it replaced to “config.inc.php_hestia”! So I renamed the new file, then renamed config.inc.php_hestia back to config.inc.php and now roundcube seems to be working normally again.
Strange. Something to keep an eye on next time anyone does a routine “apt upgrade” and roundcube is in the list of packages being upgraded.
–Dan