What a big mess! So thank you all for your work here, but for me this will be step3 or so… My problem starts with a non-accessible hassio on a rpi3. The UI is broken, AND smb and ssh are not coming up. So my chance is the base console of the hassio docker underlay… I am stuck.
Is there any chance to revert from here, or do I have to re-flash my sd-card?
F***! Why wasn’t there any warning about THAT! change?
Thanks for any help!
Bloody… Tried restoring v2 with a snapshot, creating a new special mqtt user, uninstalling, reinstalling, rebooting…
Still only getting Socket error on client <unknown>, disconnecting., my Node-red instance refuses to connect to mosquitto and whenever I try to add mqtt as a integration all I get is could not connect to broker whatever adress, username, password I try…
Is there a 0.81.7 on the way?
I don’t even use MQTT but I have similar problems loading my system… File system is fine, (intel nuc) - can access even the snapshot files, etc… but I can’t get interface to load… it either does the bootup loop issue, or it says everything is running but nothing really is… The systemd says its in degraded mode…
I have access to the backups from a day prior (hassio snapshots) and I can access them… What do I do to restore it? they’re in /usr/share/hassio/backups . and its accessible. Just not sure how to utilize them I guess… My system has never responded to “hassio …” commands… running hassio/dockmer on intel nuc. I can’t remember which install method I followed though. Had some issue when I first installed, and I tried so many diff things and variations, no idea what ended up working. Thx in advance
I am 5 minutes away from a full reflash. DAMNED! I got into the hassio by starting the docker images as root one by one. So I got back the UI, ssh and samba. I was able to kill the mqtt: section from the configuration and add a user. But the supervisor is crashing every second, so I am not able to change addons now. I cannot find any logfiles and I am pissed!
Tried again and again and can’t put MQTT v3 working… will stay on v2…can’t understand why it’s so hard since the v2 was so simple… DEVs have a look to this…
Same here…Flashed my SD three times and tried every option mentioned in this post: still no luck 
Reverting back to my old config and will stay on V2 for a while.
I changed the MQTT broker addon to the MQTT server and web client.
(https://github.com/hassio-addons/addon-mqtt/blob/v0.1.1/README.md).
It is (still) working.
OK, next step. I was able to kill the discovery.json file in hassio (root) in dir hassio:/mnt/data/resin-data/
After that the supervisor stopped crashing, and I am now able to get into mqtt itself.
Same problems over here, mqtt not working anymore, now trying to set a old snapshot back where is works
Back in business!
Login to raspi hassio Docker root
start homeassistant and ssh, smb addons by hand (docker start homeassistant / docker start addon_core_ssh / docker start addon_core_samba
Delete all mqtt: stuff from configuration.yaml (via samba)
Delete /mnt/data/resin-data/discovery.json (via root ssh docker)
docker start resin_supervisor
login at UI - configure MQTT 3 Addon with user Account
config Addon:
{
“logins”: [
{
“username”: “myuser”,
“password”: “mypass”
}
],
“anonymous”: false,
“customize”: {
“active”: false,
“folder”: “mosquitto”
},
“certfile”: “fullchain.pem”,
“keyfile”: “privkey.pem”
}
Added exact same user in settings/user: myuser, mypass
full hassio restart
configure integrations - Take a look to the top, message appears: “Mosquitto” - Do you want to use the integratet broker - yes! (here is the magic!) - Success (only with matching users)
Restart hassio (if not all mqtt devices will stay “yellow”
BACK IN BUSINESS!
I did the same thing yesterday, but I was told I would eventually have to change back. I had 96 MQTT clients needing direction, clicking relays and blinking lights all over the house…and then my wife tripped while trying to leave for work in the dark…not very happy…I took the quick fix LOL.
Same problems for me… lost 4h and eventually reverted back to V2…
This rundown matches my own experience getting it to work. One important thing that is missing is that after removing discovery.json and getting back in, don’t attempt a reboot until you have updated to Supervisor 139.
A misunderstanding I think. You don’t have to change back. The community repo MQTT addon will also start using the HA user credentials at some point (all the addons are adding this that can, so that user management can be implemented).
The difference is that the official repo mqtt addon ties in with the one-click integration. I don’t know if the community repo mqtt addon will.
Either way, the choice will remain.
PRAISE THE FLYING SPAGHETTI MONSTER!
another system is up! man, that was more than 24h living like savages [I mean: turning most of the lights by hand] but right now seems like all my mqtt switches [tasmotized] and sensors are back online.
Home Assistant 0.81.6; Supervisor 139, HassOS 1.12, Mosquitto v3; Samba v8. my steps eventually were those:
finally there were no issues with bootloop, so I skipped hunting the discovery.json file
after many fails I hanged up my RPi3, so I flashed it fresh&clean with latest and greatest hassio image
recovered my snapshot with 0.81.5 with wipe
after reading all this thread and brainstorming with my brother [his system also died yesterday] I TURNED OFF mqtt in all my ESP-based devices [lots of sonoffs basic and sonoffs touch; all with tasmota, so was relatively easy]
uninstalled mqtt broker, removed mqtt from configuration
took out the zigbee dongle & remove the zigbee2mqtt configuration
restarted & updated everything, after another reboot made sure everything is working
installed mqtt broker, entered the configuration at the addon page:
{
"logins": [
{
"username": "NEW-MQTT-USER",
"password": "NEW-MQTT-PASSWORD"
}
],
"anonymous": false,
"customize": {
"active": false,
"folder": "mosquitto"
},
"certfile": "fullchain.pem",
"keyfile": "privkey.pem"
}
entered mqtt configuration in the yaml (I’m 99,99% sure that addon config OVERWRITES that, but when my brother was testing his fixes - broker started to work ONLY after adding this to yaml):
mqtt:
broker: core-mosquitto
username: !secret mqtt_username
password: !secret mqtt_password
discovery: true
restart, checked the mqtt logs in the addons page - to be sure everything is ok)
device by device - turned on the mqtt, entered new credentials
installed zigbee dongle, added zigbee2mqtt config, restart
all sensors & switches are back!
I assume that step 9 makes no sense, but I won’t risk deleting that 
I DIDN’T add any user with the new authorisation module - only local user in the mqtt addon configuration.
my guess is, that everything worked earlier, but after changing the user all my mqtt devices were trying to connect to broker and then failed because of… DDoS! so, the more mqtts are in the system, the bigger “DDoS attack” is performed and hassio just can’t get it right. before I turned off mqtt in all tasmotas - the switches when touched were terribly slow. they were endlessly connecting with broker…
so, THANK YOU everyone involved in this rough fight. it was good to brainstorm with you all, I am sure that it helped a lot with my restoration!
and good luck to everyone who still is fighting with the dead hassio installation. THERE IS A HOPE!
Yes my hassio is up and running again.
Many thank for the options above, does the trick also for me 
What a time for me to try using MQTT for the first time 2 days ago.
My mqtt light keeps disconnecting and reconnecting repeatedly every min and I’m not sure if its related to this update or me just misconfiguring something.
I had everything setup the way @pejotigrek has it except i also have “discovery_prefix: homeassistant” in my config under mqtt:
Get rid of it!
Get rid of everything including:
mqtt:
It isn’t needed
I updated and had the problem as well.
But…
Thanks to all the back and forth on this issue. I’ve got my system sorted out and working.
Thanks all.
Then how do I enable discovery?