Connection to MQTT Broker failed "-rc 4"

[in2itiv]

Hello,

i'm trying to read out the data from a smart meter (https://www.smartcircuits.de/ - no advertising, just to clarify!).
Data is display on the smart meters web interface correctly and can be polled via HTTPS+JSON without any error from my endpoint (openDTU on battery).
But now I want to gather all my data on my Raspberry (Model 4b) with ioBroker to publish it via MQTT. Therefore i activated and configured the MQTT connection, but the connection to my mqtt broker won't be established - error: Connect failed to 192.168.179.4:1887, rc -4. Retry in 120 sec .

Since i'm already running a MQTT Broker on my Raspberry to gather other sensor data i had to switch the MQTT Port of the SonOff Adapter from default to 1887.

On the other end (broker) it reads out the sensors' name, so i assume a connection was established for a short moment. Unfortunately no sensor data was exchanged.

sonoff.0   2026-05-10 18:44:59.179  info  Client [DVES_2E6830] connection closed: closed
sonoff.0   2026-05-10 18:44:54.950  debug  stateChange sonoff.0.info.connection: {"val":"DVES_2E6830","ack":true,"ts":1778431494945,"q":0,"from":"system.adapter.sonoff.0","user":"system.user.admin","lc":1778431494945}
sonoff.0   2026-05-10 18:44:54.885  debug  stateChange sonoff.0.info.clients.DVES_2E6830: {"val":true,"ack":true,"ts":1778431494881,"q":0,"from":"system.adapter.sonoff.0","user":"system.user.admin","lc":1778431494881}
sonoff.0   2026-05-10 18:44:54.868  debug  process task: {"type":"addObject","id":"sonoff.0.DVES_2E6830.alive","data":{"_id":"sonoff.0.DVES_2E6830.alive","common":{"type":"boolean","role":"indicator.reachable","read":true,"write":false,"name":"DVES_2E6830 alive"},"type":"state"}}
sonoff.0   2026-05-10 18:44:54.865  debug  process task: {"type":"addObject","id":"sonoff.0.DVES_2E6830","data":{"_id":"sonoff.0.DVES_2E6830","common":{"name":"DVES_2E6830","desc":""},"native":{"clientId":"DVES_2E6830"},"type":"channel"}}
sonoff.0   2026-05-10 18:44:54.864  info  Client [DVES_2E6830] connected with secret 1778431494862_3551

Now the really strange part...
It worked up to last week on Tasmota 14.4.1 and stopped somehow unrecognized. Since then i had no luck getting it back to an operating state, so i updated the smart meter Tasmota version to 15.3.0. This did not solve the issue. It worked for a few hours, then suddenly stopped and won't connect properly since this afternoon - resulting in the same error msg mentioned above (rc -4)

I'm expecting a sensor value string like this (extract from https+json).

17:42:56.616 RSL: SENSOR = {"Time":"2026-05-10T17:42:56","SML":{"Verbrauch_T1":0.2280229,"Verbrauch_T2":10156.2892004,"Energy":10156.5172233,"Einspeisung_Summe":640.7091505,"Watt_L1":161.69,"Watt_L2":74.15,"Watt_L3":224.59,"Power":460.44,"Volt_L1":234.0,"Volt_L2":233.8,"Volt_L3":233.3}}

by the way... it's the same error msg connecting to sonoff adapter or any other mqtt-broker, no matter if mosquitto from homeassistant (already tried..), mqtt from ioBroker or sonoff.

Thanks for any hints in advance!

[sfromis]

As documented, the meaning of rc -4 is "the server didn't respond within the keepalive time". Thus the issue is outside of Tasmota, related to the connection to your broker, or it being slow to respond. There are commands to tweak MQTT-related timeout values.

[in2itiv]

Thanks for your reply. I tweaked the values of MqttWifiTimeout and MqttTimeout since i assumed the issue might be solved.. But it was no success. If i push the value to far, i got problems reaching the console via webinterface.
Also tried to setup the Mqtt Connection from scratch, but it all results in the same connection failed rc -4 state.

The most irritating thing about that connection is, that the last value i received in ioBroker has a timestamp from 2026-05-13 00:04:34 ... so it worked for brief moment and sent some data..

[sfromis]

Did you try setting MqttKeepAlive as suggested in the return code meaning I quoted?

[in2itiv]

Yes, i also tweaked that value up and also down, but i still got the same error msg.
However the sensor send one more value from today... Vasily, give me one ping. only one...