SquirrelMail - Error: Connection dropped by IMAP server

I am trying to use SquirrelMail on Ubuntu 16.04.

When I go to localhost/squirrelmail/src/login.php and login, I encounter the error message: ERROR: Connection dropped by IMAP server.

I tried the solution here, which says to add in /etc/dovecot/dovecot.conf:

protocol imap {
mail_location = mbox:~/mail:INBOX=/var/mail/%u
}

but the error persists.

Additionally, I tried the solution here, which informs to add in /etc/dovecot/dovecot.conf:

namespace inbox {
    inbox = yes
}

but again, the error persists.

My /etc/dovecot/dovecot.conf file is:

## Dovecot configuration file
# If you're in a hurry, see http://wiki2.dovecot.org/QuickConfiguration
# "doveconf -n" command gives a clean output of the changed settings. Use it
# instead of copy&pasting files when posting to the Dovecot mailing list.
# '#' character and everything after it is treated as comments. Extra spaces
# and tabs are ignored. If you want to use either of these explicitly, put the
# value inside quotes, eg.: key = "# char and trailing whitespace  "
# Most (but not all) settings can be overridden by different protocols and/or
# source/destination IPs by placing the settings inside sections, for example:
# protocol imap { }, local 127.0.0.1 { }, remote 10.0.0.0/8 { }
# Default values are shown for each setting, it's not required to uncomment
# those. These are exceptions to this though: No sections (e.g. namespace {})
# or plugin settings are added by default, they're listed only as examples.
# Paths are also just examples with the real defaults being based on configure
# options. The paths listed here are for configure --prefix=/usr
# --sysconfdir=/etc --localstatedir=/var
# Enable installed protocols
!include_try /usr/share/dovecot/protocols.d/*.protocol
# A comma separated list of IPs or hosts where to listen in for connections. 
# "*" listens in all IPv4 interfaces, "::" listens in all IPv6 interfaces.
# If you want to specify non-default ports or anything more complex,
# edit conf.d/master.conf.
#listen = *, ::
# Base directory where to store runtime data.
#base_dir = /var/run/dovecot/
# Name of this instance. In multi-instance setup doveadm and other commands
# can use -i <instance_name> to select which instance is used (an alternative
# to -c <config_path>). The instance name is also added to Dovecot processes
# in ps output.
#instance_name = dovecot
# Greeting message for clients.
#login_greeting = Dovecot ready.
# Space separated list of trusted network ranges. Connections from these
# IPs are allowed to override their IP addresses and ports (for logging and
# for authentication checks). disable_plaintext_auth is also ignored for
# these networks. Typically you'd specify your IMAP proxy servers here.
#login_trusted_networks =
# Space separated list of login access check sockets (e.g. tcpwrap)
#login_access_sockets = 
# With proxy_maybe=yes if proxy destination matches any of these IPs, don't do
# proxying. This isn't necessary normally, but may be useful if the destination
# IP is e.g. a load balancer's IP.
#auth_proxy_self =
# Show more verbose process titles (in ps). Currently shows user name and
# IP address. Useful for seeing who are actually using the IMAP processes
# (eg. shared mailboxes or if same uid is used for multiple accounts).
#verbose_proctitle = no
# Should all processes be killed when Dovecot master process shuts down.
# Setting this to "no" means that Dovecot can be upgraded without
# forcing existing client connections to close (although that could also be
# a problem if the upgrade is e.g. because of a security fix).
#shutdown_clients = yes
# If non-zero, run mail commands via this many connections to doveadm server,
# instead of running them directly in the same process.
#doveadm_worker_count = 0
# UNIX socket or host:port used for connecting to doveadm server
#doveadm_socket_path = doveadm-server
# Space separated list of environment variables that are preserved on Dovecot
# startup and passed down to all of its child processes. You can also give
# key=value pairs to always set specific settings.
#import_environment = TZ
##
## Dictionary server settings
##
# Dictionary can be used to store key=value lists. This is used by several
# plugins. The dictionary can be accessed either directly or though a
# dictionary server. The following dict block maps dictionary names to URIs
# when the server is used. These can then be referenced using URIs in format
# "proxy::<name>".
dict {
  #quota = mysql:/etc/dovecot/dovecot-dict-sql.conf.ext
  #expire = sqlite:/etc/dovecot/dovecot-dict-sql.conf.ext
}
# Most of the actual configuration gets included below. The filenames are
# first sorted by their ASCII value and parsed in that order. The 00-prefixes
# in filenames are intended to make it easier to understand the ordering.
!include conf.d/*.conf
# A config file can also tried to be included without giving an error if
# it's not found:
!include_try local.conf

My /etc/postfix/main.cf is:

# See /usr/share/postfix/main.cf.dist for a commented, more complete version
# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no
# appending .domain is the MUA's job.
append_dot_mydomain = no
# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h
readme_directory = no
# TLS parameters
smtpd_tls_cert_file = /etc/postfix/ssl/mailserver.crt
smtpd_tls_key_file = /etc/postfix/ssl/mailserver.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = ubuntu-vm.localdomain
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = $myhostname, localhost, ubuntu-vm, localhost.localdomain, localhost
relayhost = 
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all
smtpd_sasl_local_domain = 
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination
smtp_tls_security_level = may
smtpd_tls_security_level = may
smtpd_tls_auth_only = no
smtp_tls_note_starttls_offer = yes
smtpd_tls_CAfile = /etc/postfix/ssl/cacert.pem
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom
localhost = example.com

In addition, I went to localhost/squirrelmail/src/configtest.php and received:

SquirrelMail configtest
This script will try to check some aspects of your SquirrelMail configuration and point you to errors whereever it can find them. You need to go run conf.pl in the config/ directory first before you run this script.
SquirrelMail version:   1.4.23 [SVN]
Config file version:    1.4.0
Config file last modified:  01 July 2020 20:51:58
Checking PHP configuration...
    PHP version 7.0.33-0ubuntu0.16.04.15 OK.
    Running as www-data(33) / www-data(33)
    display_errors:
    error_reporting: 22527
    variables_order OK: GPCS.
    PHP extensions OK. Dynamic loading is disabled.
    ERROR: You have configured PHP not to allow short tags (short_open_tag=off). This shouldn't be a problem with SquirrelMail or any plugin coded coded according to the SquirrelMail Coding Guidelines, but if you experience problems with PHP code being displayed in some of the pages and changing setting to "on" solves the problem, please file a bug report against the failing plugin. The correct contact information is most likely to be found in the plugin documentation.
Checking paths...
    Data dir OK.
    Attachment dir OK.
    Plugins OK.
    Themes OK.
    Default language OK.
    Base URL detected as: http://localhost/squirrelmail/src (location base autodetected)
Checking outgoing mail service....
    SMTP server OK (220 ubuntu-vm.localdomain ESMTP Postfix (Ubuntu))
Checking IMAP service....
    IMAP server ready (* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE AUTH=PLAIN] Dovecot ready.)
    Capabilities: * CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE AUTH=PLAIN
Checking internationalization (i18n) settings...
     gettext - Gettext functions are available. On some systems you must have appropriate system locales compiled.
     mbstring - Mbstring functions are unavailable. Japanese translation won't work.
     recode - Recode functions are unavailable.
     iconv - Iconv functions are available.
     timezone - Webmail users can change their time zone settings.
Checking database functions...
    not using database functionality.
Congratulations, your SquirrelMail setup looks fine to me!
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