netplan + WiFi: link is not ready (18.04 Server)

Question

I'm trying 18.04 Server with this fancy new netplan and can't get my WiFi to work. Doing it the old-fashioned way (e.g. configuring wpa_supplicant.service and using dhclient), everything is fine. But with netplan I get syslog messages saying IPv6: ADDRCONF(NETDEV_UP): wlp2s0: link is not ready and networkctl has interface wlp2s0 as "no-carrier" and "configuring". Here's the relevant part of my 01-netcfg.yaml file:

[Edit, see update below]

What am I missing here?

Thanks in advance

Edit1, due to comments ======================

[Edit, see update below]

So chili555 seems to be wrong about IPv6, error persists even with static config. And George, the debug output looks OK to me. Also, the gateway should not be an issue right now, but yes, I use the last IP in the network...

Output of netplan --debug apply

** (generate:3281): DEBUG: 19:10:34.892: Processing input file //etc/netplan/01-netcfg.yaml..
** (generate:3281): DEBUG: 19:10:34.892: starting new processing pass
** (generate:3281): DEBUG: 19:10:34.892: wlp2s0: adding wifi AP 'MY_ESSID'
** (generate:3281): DEBUG: 19:10:34.892: enp3s0: setting default backend to 1
** (generate:3281): DEBUG: 19:10:34.892: wlp2s0: setting default backend to 1
** (generate:3281): DEBUG: 19:10:34.892: Generating output files..
** (generate:3281): DEBUG: 19:10:34.893: NetworkManager: definition enp3s0 is not for us (backend 1)
** (generate:3281): DEBUG: 19:10:34.893: wlp2s0: Creating wpa_supplicant configuration file run/netplan/wpa-wlp2s0.conf
** (generate:3281): DEBUG: 19:10:34.893: Creating wpa_supplicant service enablement link /run/systemd/system/multi-user.target.wants/netplan-wpa@wlp2s0.service
** (generate:3281): DEBUG: 19:10:34.893: NetworkManager: definition wlp2s0 is not for us (backend 1)
DEBUG:netplan generated networkd configuration exists, restarting networkd
DEBUG:no netplan generated NM configuration exists
DEBUG:device lo operstate is unknown, not replugging
DEBUG:netplan triggering .link rules for lo
DEBUG:replug wlp2s0: unbinding 0000:02:00.0 from /sys/bus/pci/drivers/iwlwifi
DEBUG:replug wlp2s0: rebinding 0000:02:00.0 to /sys/bus/pci/drivers/iwlwifi
DEBUG:device enp3s0 operstate is up, not replugging
DEBUG:netplan triggering .link rules for enp3s0

=== Edit 2 due to comments

New .yaml fragment with dhcp4/6 ABOVE addresses, even though it works fine for Ethernet. Still in networkctl: No-carrier/configuring

  wifis:
    wlp2s0:
      dhcp4: false
      dhcp6: false
      addresses:
        - 172.20.12.195/24
        - fd13:4630:2901:12::195/64
      gateway4: 172.20.12.254
      access-points:
        "MyESSID":
          password: "MyPassword"

Also, sorry, but honestly: WTF? "Watch your indentation"? Where are we, grammar school? Is it supposed to stay like this? Especially since the (alternate) installer doesn't even give me the chance to configure networking during the installation if it detects DHCP...

=== Edit 3 due to sheer frustration

Just reinstalled (maybe my manual tinkering broke something) and took the setup-generated .yaml file as a template, so now it looks like this:

network:
  version: 2
  renderer: networkd
  ethernets:
    enp3s0:
      dhcp4: yes
  wifis:
    wlp2s0:
      dhcp4: yes
      access-points:
        "MyESSID":
          password: "MyPassword"

Still no luck, so please stop talking about the order of entries and indentation and stuff, I think it's just plain broken. At least in the alternate installer version of 18.04 server. Next stop: live-server install...