Comment 3 for bug 1874730

The log shows there was a configuration question for postfix, that caused apache2 to be left unconfigured:

Setting up postfixadmin (3.2.1-3) ...
Installing new version of config file /etc/apache2/conf-available/postfixadmin.conf ...

Configuration file '/etc/postfixadmin/config.inc.php'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** config.inc.php (Y/I/N/O/D/Z) [default=N] ?
Package apache2 is not configured yet. Will defer actions by package postfixadmin.
[...]

Setting up phppgadmin (7.12.1+dfsg-1) ...
Installing new version of config file /etc/phppgadmin/config.inc.php ...
Package apache2 is not configured yet. Will defer actions by package phppgadmin.
[...]

Setting up apache2 (2.4.41-4ubuntu3) ...
info: Executing deferred 'a2enconf phppgadmin' for package phppgadmin
ERROR: Conf phppgadmin does not exist!
dpkg: error processing package apache2 (--configure):
 installed apache2 package post-installation script subprocess returned error exit status 1