==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
downloadable installer? (bash iRedMail.sh)
MariaDB backend
Nginx
====

Errors were encountered while processing:
nginx-full
E: Sub-process /usr/bin/dpkg returned an error code (1)

So I scrolled up somewhat and see:

invoke-rc.d: initscript nginx, action "start" failed.
* nginx.service - A high performance web server and a reverse proxy server
   Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Mon 2019-03-25 14:32:26 CET; 15ms ago
     Docs: man:nginx(8)
  Process: 19395 ExecStart=/usr/sbin/nginx -g daemon on; master_process on; (code=exited, status=1/FAILURE)
  Process: 19393 ExecStartPre=/usr/sbin/nginx -t -q -g daemon on; master_process on; (code=exited, status=0/SUCCESS)

Mar 25 14:32:24 mail nginx[19395]: nginx: [emerg] listen() to [::]:80, back…use)
Mar 25 14:32:25 mail nginx[19395]: nginx: [emerg] listen() to 0.0.0.0:80, b…use)
Mar 25 14:32:25 mail nginx[19395]: nginx: [emerg] listen() to [::]:80, back…use)
Mar 25 14:32:25 mail nginx[19395]: nginx: [emerg] listen() to 0.0.0.0:80, b…use)
Mar 25 14:32:25 mail nginx[19395]: nginx: [emerg] listen() to [::]:80, back…use)
Mar 25 14:32:26 mail nginx[19395]: nginx: [emerg] still could not bind()
Mar 25 14:32:26 mail systemd[1]: nginx.service: Control process exited, cod…us=1
Mar 25 14:32:26 mail systemd[1]: Failed to start A high performance web ser…ver.
Mar 25 14:32:26 mail systemd[1]: nginx.service: Unit entered failed state.
Mar 25 14:32:26 mail systemd[1]: nginx.service: Failed with result 'exit-code'.
Hint: Some lines were ellipsized, use -l to show in full.
dpkg: error processing package nginx-full (--configure):

Is there some IPv6 problem?

I have the 127.0.0.1 line as per documentation, but in addition to that I have

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

in /etc/hosts, because that's what Debian comes with and there were no explicit instructions to delete these lines.

So what did break?

Edit:

For whatever reason, Apache was running so Nginx couldn't bind to port 80. A little consistency check and/or graceful abort wouldn't hurt - I guess.