All working
Just at startup when the screen boots, the first line in proeminent and the other not so
Is quite cool the simbol generator 
Wow. All those bits look perfect. Then I believe the problem is a character set... if only my brain would cooperate... sry. Searching...
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Found something about a different chip set... (it references the WOKWI simulator):
The LCD1602 uses the Hitachi HD44780 LCD controller chip. The chip comes with a built-in font and the ability to define up to 8 custom characters.
There are two versions of the chip's ROM with two different fonts: HD44780UA00, Japanese katakana characters, and HD44780UA02, which includes Western European characters.
Wokwi simulates the HD44780UA00 variant. It has a total of 256 characters:
You can see that the first eight characters are user-defined. It allows you to create custom shapes and store them. You will see how to create custom characters and load them in your following Arduino projects. Let us start with a basic example.
But if the code would be wrong and display something odd then it shouldn t work with the laptop usb power then. I was thinking that the reading of the termistor would cause a bad reading and exced the 16 spaces but it doesn t seem to, if it would what would happen, just ignore them ? I have modified lcd.print(" C ")
so that i won t get triple C C C for small degrees but overide eith empty space
It does work sometimes so it is a combination of japaneze and european characters, i am using print instead of write wich transforms the text into "string"to display on screen, even the integer numbers of temperature
Can you leave it in its normal work place with your test pattern(s) either statically or dynamically displayed? See if it does the same as before.
On the back of the LCD are two black blobs. The bigger of the two black blobs on the back of the LCD1602 protects the HD44780. The 80 pads of the HD44780 are 100 micrometers square (0.1mm). One or some pads could be prone to lifting if the board flexes from mounting hardware, heat, cold, or vibration... keeping it comfortable might help. I wouldn't know how or where to replace the HD44780, other than an engineering company who have surface mount equipment.
Your character set!
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i have made more tests and it turns out that if i use any 5v charger/psu and power the nano from it's usb port, they all work (so far) and i didn't got any issues. If i try to use them on that round port wich is conected to arduino 5v bar directly and all other component's positive side, it malfuntions, doesn't show anything and even made the arduino not working again until i have reuploaded the sketch..
About the lcd, it is true that i have kind of forced it's black cover in place in the plastic mount as it was a snug fit, could that press on something?
Nice datasheet, but more than my understanding capabilites
If the chip in question was soldered imperfectly, yes. Try to relieve the stress. The option to "reflow" the chip is more than buying a new 1602.
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have you soldered the I2C BackPack to the LCD? Can you please show the soldering?
It will not work reliable, if you just stick the backpack in the LCD. You MUST solder it.
And just in case it was not already told: an I2C bus with wires just beside a 230V AC line can cause problems.
the i2c board it is soldered very good in the screen holes, all connections are fully penetrated with solder, there shouldn't be any discontinuity
About your second note, indeed the 230v ac wires that come trough that black cable and goes to the relay then in the socket are passing under the screen but the problem shows even if the ac cable is unpluged for wich comes the difference betwen usb or external supply via round plug in panel. Just realised that the 5v cable of the external psu also pass right next to i2c cable of the screen, paralel to them. It s a it of mess inside so that might actualy have a noise effect besides the ssr wich works with usb because it is just on top of the whole enclosure verry close to the edge where the arduino is placed so it doesn t get to come near other wires.
Since there is a bit of space inside but still the wires are close and all over the place, of i were to distance them or use shielding, should i wire it to - terminal of the 5v psu or the earth ground of the 230v cable ? For ex. a usb cable has the shielding conected to the outer metal part of the usb conector wich trasnsfers to the pc case wich is conected to ground but is it also conected to - ? Because the motherboard has some metal pads that are screwed to the case, i suppose they are in place for a reason of noise filtering ?
Ground RFI case to next ground in line (shielded cable, another case), it will eventually get to earth.
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