Good day everyone,
in case someone faces the same issues I did:

• controlling multiple relays from one arduino and sharing the same power (powering relay board from Arduino
  or
• controlling relay which switches high loads circuit causing connected LCD to show garbage and again powering the relay from Arduino...

...the solution proposed above is the only one which helps - provide separated power supply for the relay boards. Once this is done with an extra little bucket power source for 5V (3W) all the issues with relays not working, domoticz not switching relays, LCD going crazy - all solved. I should have done this long before.

Thanks everyone!

P.S. For those, who still do not follow, the relay boards for Arduino contain extra jumper connecting VCC and JD-VCC (plus sometimes extra GND pin). So remove the jumper and connect the standalone, not common sharing 5V power source to this extra JD-VCC and GND (or GND near the IN1 pin) and disconnect the GND from Arduino and then you have a full optosiolation and then there are no interferences and other garbage breaking the Arduino functionality by switching relays coils. (also explained here: https://forum.arduino.cc/index.php?topic=470134.0)

I tell you guys, I have about 10 nodes at home with relays, some switching lights, some doors, some 12 motors. All I had to do was to provide separate source to the relay from the source to the arduino. Since then, last 2 years I get 98% reliability.

@skywatch, thanks for the reply. I read about using capacitaators, but never in such a complex and straightforward way. I can surely try it as well and switch back to one power supply and see "the difference".
I look forward your tests and findings, if you can post them here. Is there any "formula" to calculate the capacitor values - when you say 100 uF or more - what can you propose as more and what is already too much?
Also when you say "good ceramic or electrolytic" - what do you refer to as "good"?
Basically if I read you well, it is the same as having the capacitors on NRF24 radio on + and GND to avoid issues with signal glitches?
Thanks again to take time to reply here. Really appreciate this kind of feedback!

I tell you guys, I have about 10 nodes at home with relays, some switching lights, some doors, some 12 motors. All I had to do was to provide separate source to the relay from the source to the arduino. Since then, last 2 years I get 98% reliability.

@gohan OK, then this one I never saw in logs doing anything, or it perhaps actually never worked for me

@gohan Hmm, then I dare to say I have never seen such entry. I could see in the past (before I added second power supply for relay boards) many information about repeating last command and lately issues with sending gmail notifications (caused by securing gmail account) but I do not recall seeing an information about node restart and by that I mean even manual restarts I did by unplugging the node. But I guess you refer to watchdog driven from the controller - Domoticz Gateway. I was perhaps answering on another "watchdog" - a internal Arduino node code which monitors the Arduino internally and checks whether it is running and if not, then it restarts itself. I cant imagine a remote restart of a node which is not responding to commands, how could it react on a remote restart? I suppose it can work just in case the arduino works, only the node itself is somehow not reporting data or so, right?

@gohan I have tried all kinds of things. I believe that the fact that I used watchdogs which checked the MySensors nodes every 8 seconds (or less) helped me to recover from relay failures so the nodes appeared to be functional, but only randomly. In real life it looked good when I started them. did several turn on and off and it worked. In 2-3 minutes I did try again, it worked for 2 out of 4 attempts and then it stopped responding. In another 20-30 seconds I did try again and then again it appeared to work - I believe it was thanks to the watchdogs recovering the frozen node after the relay peak caused them to stop working. But only a theory, I have no deeper knowledge to prove this.

@parachutesj I do say it does. I have about 50 nodes in my house, 15 relays or so, temperature sensors and some others. All the relays i power now with standalone little china made 5V, 3W adapters and as I sit now in my house I do switch on and off lights, turn on gate, control pumps in greenhouse, all works on the first tap. I am finally happy with my mysensors equipped domoticz, first after 2 years. It took my so long to just give it a try.

Good day everyone,
in case someone faces the same issues I did:

• controlling multiple relays from one arduino and sharing the same power (powering relay board from Arduino
  or
• controlling relay which switches high loads circuit causing connected LCD to show garbage and again powering the relay from Arduino...

...the solution proposed above is the only one which helps - provide separated power supply for the relay boards. Once this is done with an extra little bucket power source for 5V (3W) all the issues with relays not working, domoticz not switching relays, LCD going crazy - all solved. I should have done this long before.

Thanks everyone!

P.S. For those, who still do not follow, the relay boards for Arduino contain extra jumper connecting VCC and JD-VCC (plus sometimes extra GND pin). So remove the jumper and connect the standalone, not common sharing 5V power source to this extra JD-VCC and GND (or GND near the IN1 pin) and disconnect the GND from Arduino and then you have a full optosiolation and then there are no interferences and other garbage breaking the Arduino functionality by switching relays coils. (also explained here: https://forum.arduino.cc/index.php?topic=470134.0)

@skywatch P.S. I have one more theory I need to prove. Based on another discussion where my sauna control display fails to display properly after several relay switch on and off which according to many users is caused by the use of common power source for arduino/LCD and the relays which create short but noticeable power peaks. So I will try to power my 8 node relay board from a independent power source from the power used for arduino... A little chance, but worth testing.

Hi skywatch,
would you be so kind and share your ack and heart beat code snippets? I tried to use the code from examples, but it seems teh heartbeat never worked as expected. Regarding the ack - I am getting constantly a message: Error: MySensors: Repeating previous command (2/2) - seems to me to be related to ack as it simply does not get acknowledgement from the node and repeats the command again. But in 99% cases the command seems to be executed fine.

@skywatch thanks for the answer. I did compare all and they all share teh same code. Pretty much. Some work for year or two without being touched and they are even further away from the gateway. I do think, that there is either HW problem with the arduino or the radio module. Before I touch it - which of those two determine what ID is the sensor? Is it the Radio module or arduino? I ask because when I change the one which delivers the ID, all the relays on the controller will be lost and I will have to rewrite all related dependencies etc.

Regarding your further not on the lost configuration. I found somewhere a piece of code - a script, which does upload daily the current full database and configuration to dropbox - so in case I loose the raspberry Pi or SD card, I can restore it in a minute with the history. If you ned I can find it and send it.