I am able to connect to wifi networks that don't require password.
But when I try to connect to a network that requires password, the password is not asked, Network Manager just tries to connect and of course it fails.

This is the log from /var/log/syslog:

NetworkManager[936]: <info>  [1495625898.1943] device (wlp2s0): Activation: starting connection 'true_home2G_b68' (ea2f25dd-98bd-4e72-b660-5ac56c683519)
NetworkManager[936]: <info>  [1495625898.1944] audit: op="connection-activate" uuid="ea2f25dd-98bd-4e72-b660-5ac56c683519" name="true_home2G_b68" pid=2168 uid=1000 result="success"
NetworkManager[936]: <info>  [1495625898.1945] device (wlp2s0): state change: disconnected -> prepare (reason 'none') [30 40 0]
NetworkManager[936]: <info>  [1495625898.1946] manager: NetworkManager state is now CONNECTING
NetworkManager[936]: <info>  [1495625898.1951] device (wlp2s0): state change: prepare -> config (reason 'none') [40 50 0]
NetworkManager[936]: <info>  [1495625898.1953] device (wlp2s0): Activation: (wifi) access point 'true_home2G_b68' has security, but secrets are required.
NetworkManager[936]: <info>  [1495625898.1953] device (wlp2s0): state change: config -> need-auth (reason 'none') [50 60 0]
NetworkManager[936]: <warn>  [1495625923.2170] device (wlp2s0): No agents were available for this request.
NetworkManager[936]: <info>  [1495625923.2171] device (wlp2s0): state change: need-auth -> failed (reason 'no-secrets') [60 120 7]
gnome-session[1940]: (nm-applet:2168): GLib-GObject-CRITICAL **: g_object_unref: assertion 'G_IS_OBJECT (object)' failed
NetworkManager[936]: <info>  [1495625923.2174] manager: NetworkManager state is now DISCONNECTED
NetworkManager[936]: <warn>  [1495625923.2184] device (wlp2s0): Activation: failed for connection 'true_home2G_b68'
NetworkManager[936]: <info>  [1495625923.2199] device (wlp2s0): state change: failed -> disconnected (reason 'none') [120 30 0]
kernel: [32456.473864] IPv6: ADDRCONF(NETDEV_UP): wlp2s0: link is not ready

I don't know if it matters, I think this problem started after installation of anbox. After that, in ifconfig results, wlp2s0 was replaced by another entry (anboxlo if I remember correctly). Now I removed anbox and tried to put back the old settings, searched for the errors in output… but no success.

Please help me here. I will be available to give more information as required.

EDIT 1 to add information required by @George in comment:

Result of $ less /var/log/syslog | grep -i wpa:

May 28 15:11:25 dv-XPS-15-9550 dbus[888]: [system] Activating via systemd: service name='fi.w1.wpa_supplicant1' unit='wpa_supplicant.service'
May 28 15:11:25 dv-XPS-15-9550 systemd[1]: Starting WPA supplicant...
May 28 15:11:25 dv-XPS-15-9550 dbus[888]: [system] Successfully activated service 'fi.w1.wpa_supplicant1'
May 28 15:11:25 dv-XPS-15-9550 systemd[1]: Started WPA supplicant.
May 28 15:11:25 dv-XPS-15-9550 wpa_supplicant[2633]: Successfully initialized wpa_supplicant
May 28 15:11:25 dv-XPS-15-9550 NetworkManager[2619]: <info>  [1495977085.7349] supplicant: wpa_supplicant running
May 28 15:11:25 dv-XPS-15-9550 wpa_supplicant[2633]: dbus: wpa_dbus_get_object_properties: failed to get object properties: (none) none
May 28 15:11:25 dv-XPS-15-9550 wpa_supplicant[2633]: dbus: Failed to construct signal
May 28 15:11:25 dv-XPS-15-9550 wpa_supplicant[2633]: Could not read interface p2p-dev-wlp2s0 flags: No such device
May 28 15:11:33 dv-XPS-15-9550 wpa_supplicant[2633]: p2p-dev-wlp2s0: CTRL-EVENT-REGDOM-CHANGE init=BEACON_HINT type=UNKNOWN
May 28 15:13:07 dv-XPS-15-9550 wpa_supplicant[2633]: nl80211: deinit ifname=p2p-dev-wlp2s0 disabled_11b_rates=0
May 28 15:13:07 dv-XPS-15-9550 wpa_supplicant[2633]: nl80211: deinit ifname=wlp2s0 disabled_11b_rates=0
May 28 15:13:24 dv-XPS-15-9550 NetworkManager[2755]: <info>  [1495977204.9381] supplicant: wpa_supplicant running
May 28 15:13:24 dv-XPS-15-9550 wpa_supplicant[2633]: dbus: wpa_dbus_get_object_properties: failed to get object properties: (none) none
May 28 15:13:24 dv-XPS-15-9550 wpa_supplicant[2633]: dbus: Failed to construct signal
May 28 15:13:24 dv-XPS-15-9550 wpa_supplicant[2633]: Failed to create interface p2p-dev-wlp2s0: -16 (Device or resource busy)
May 28 15:13:24 dv-XPS-15-9550 wpa_supplicant[2633]: nl80211: Failed to create a P2P Device interface p2p-dev-wlp2s0
May 28 15:13:24 dv-XPS-15-9550 wpa_supplicant[2633]: P2P: Failed to enable P2P Device interface

(Network Manager seems to not be running after system boot. I run these commands: $ sudo service network-manager restart at 15:11:25, $ sudo service network-manager stop at 15:13:07 and $ sudo service network-manager start at 15:13:24)

EDIT 2 for more requested information:

$ sudo systemctl list-unit-files --type=service | grep enabled

accounts-daemon.service                    enabled 
anacron-resume.service                     enabled 
anacron.service                            enabled 
autovt@.service                            enabled 
avahi-daemon.service                       enabled 
bluetooth.service                          enabled 
brltty.service                             enabled 
cron.service                               enabled 
cups-browsed.service                       enabled 
cups.service                               enabled 
dbus-org.bluez.service                     enabled 
dbus-org.freedesktop.Avahi.service         enabled 
dbus-org.freedesktop.ModemManager1.service enabled 
dbus-org.freedesktop.thermald.service      enabled 
display-manager.service                    enabled 
dns-clean.service                          enabled 
friendly-recovery.service                  enabled 
getty@.service                             enabled 
gpu-manager.service                        enabled 
lightdm.service                            enabled 
mariadb.service                            enabled 
ModemManager.service                       enabled 
mysql.service                              enabled 
mysqld.service                             enabled 
networking.service                         enabled 
NetworkManager-wait-online.service         enabled 
pppd-dns.service                           enabled 
resolvconf.service                         enabled 
rsyslog.service                            enabled 
snapd.autoimport.service                   enabled 
snapd.service                              enabled 
snapd.system-shutdown.service              enabled 
syslog.service                             enabled 
systemd-timesyncd.service                  enabled 
thermald.service                           enabled 
ufw.service                                enabled 
unattended-upgrades.service                enabled 
ureadahead.service                         enabled 
whoopsie.service                           enabled

$ iw list | grep -i p2p

 * P2P-client
 * P2P-GO
 * P2P-device
 * start_p2p_device
 * P2P-client: 0x00 0x10 0x20 0x30 0x40 0x50 0x60 0x70 0x80 0x90 0xa0 0xb0 0xc0 0xd0 0xe0 0xf0
 * P2P-GO: 0x00 0x10 0x20 0x30 0x40 0x50 0x60 0x70 0x80 0x90 0xa0 0xb0 0xc0 0xd0 0xe0 0xf0
 * P2P-device: 0x00 0x10 0x20 0x30 0x40 0x50 0x60 0x70 0x80 0x90 0xa0 0xb0 0xc0 0xd0 0xe0 0xf0
 * P2P-client: 0x40 0xd0
 * P2P-GO: 0x00 0x20 0x40 0xa0 0xb0 0xc0 0xd0
 * P2P-device: 0x40 0xd0
 * #{ managed } <= 1, #{ P2P-device } <= 1, #{ P2P-client, P2P-GO } <= 1,
 * #{ managed } <= 1, #{ AP } <= 1, #{ P2P-client } <= 1, #{ P2P-device } <= 1,

EDIT 3 after enabling Network Manager service:

$ sudo systemctl list-unit-files --type=service | grep enabled
accounts-daemon.service                    enabled 
anacron-resume.service                     enabled 
anacron.service                            enabled 
autovt@.service                            enabled 
avahi-daemon.service                       enabled 
bluetooth.service                          enabled 
brltty.service                             enabled 
cron.service                               enabled 
cups-browsed.service                       enabled 
cups.service                               enabled 
dbus-org.bluez.service                     enabled 
dbus-org.freedesktop.Avahi.service         enabled 
dbus-org.freedesktop.ModemManager1.service enabled 
dbus-org.freedesktop.nm-dispatcher.service enabled 
dbus-org.freedesktop.thermald.service      enabled 
display-manager.service                    enabled 
dns-clean.service                          enabled 
friendly-recovery.service                  enabled 
getty@.service                             enabled 
gpu-manager.service                        enabled 
lightdm.service                            enabled 
mariadb.service                            enabled 
ModemManager.service                       enabled 
mysql.service                              enabled 
mysqld.service                             enabled 
network-manager.service                    enabled 
networking.service                         enabled 
NetworkManager-dispatcher.service          enabled 
NetworkManager-wait-online.service         enabled 
NetworkManager.service                     enabled 
pppd-dns.service                           enabled 
resolvconf.service                         enabled 
rsyslog.service                            enabled 
snapd.autoimport.service                   enabled 
snapd.service                              enabled 
snapd.system-shutdown.service              enabled 
syslog.service                             enabled 
systemd-timesyncd.service                  enabled 
thermald.service                           enabled 
ufw.service                                enabled 
unattended-upgrades.service                enabled 
ureadahead.service                         enabled 
whoopsie.service                           enabled

$ iw list | grep -i p2p
(same)

Also let me add that now when I try to connect to a wifi with password I get this error message in popup:

Connection failure

Connection activation failed

(2) Active connection removed before it was initialized