Thank you for your answer I will look at it when I get home because I’m driving in the moment. 
And it’s pretty annoying that my test that I wanted to do with the second drawing on the right side it didn’t work out at all. I wanted to do this testing just for a couple of days but this setup drains my battery basically hour by hour so fast so I’ve disconnected everything.
However I need to change the connections between the panels on the roof this is how my setup looks like:
What was the reasoning for the right hand configuration? Was it just a case of it is the easiest way to connect the third panel?
No. I disassembled the left one before I set up the whole left system and connect it because I was curious and wanted to compare the two to each other later. I’m a woman and I’m curious by nature so don’t search for any logic in this action. 
But the right side configuration for some reason didn’t work out at all because I am draining more (without any load connected to the inverter) than gaining even though today is a very sunny day.
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So. I’ve disconnected the third panel and connected 2 x 200W panels in series to the controller., the two producing only 15-20 W on an autumn sunny day. Something is still using more power than the panels can produce and I have no clue what can be without any load on the system. Truly don’t understand what is happening and I started to think that panels are not working properly or one of the diode in the panels is out or both diodes are out or I have no clue. I ran out of ideas. How I can test which part of my system is failing or connected in a wrong way?
I think your curiosity may have killed your panels. I’m assuming the two panels are both in full sun. You could try connecting just one panel at a time to determine if just one has failed…
No, I didn’t. Or not yet.
…so I’ve ordered a 4th panel. As soon as it arrives my plan is to go with the 2x2 panel in series then the two sets willconnect in parallel. In that case hopefully won’t have problem with the acceptable reverse current of the panel. Ordered a 100/50 victron mppt as well and will switch to it from the 100/30.
Until these arrive, yesterday I switched to connect all 3 panels to to series. One of the three panel is in a different angle that causes probably some perfomance problems during the sunny hours, otherwise they are doing the job nicely on a sunny day.
My concern about the battery drainage is still exist: with no any load on the system as soon as the sun goes lower on the horizont (14-15 afternoon, so not even in the early evening), the daily ‘gained’ voltage of the battery starts to drop like crazy. So…the only way keep the SOC is to turn off the panels with the DC solar switch. So I do that. But…it’s weird. Not sure how to solve that. Today it nicely went up from 13.12V to 13.46V then I forgot to turn off and when I did it was only on 13.25V. What occurs the backflow or the draining?
I have 8 panels on the roof of my cottage, of which only 4 are of the same type. Due to partial shading, they have been connected in parallel for quite a few years, because this is the only way to achieve the maximum MPPT charging current, which is 100A. Series connection is useless for me, because I don’t even reach 60A. If I were you, I would connect the panels in parallel, but it is true that your panels only have 20V and their amperage may drop under load.
This is odd. The batteries draining after sundown, unless the DC isolator is turned OFF, suggests the batteries are backfeeding into the solar panels but this cannot be the case. The second screenshot shows the battery voltage at over 13V which is fine but also the array voltage is zero which is also correct and suggests the batteries are not backfeeding into the panels. We’ll have to see if anything changes with the new MPPT controller.
Mismatched panels will generally result in poor performance.
The voltage is zero because I already shut down the connection between the battery and the panels.
The 13.25 would be fine BUT I’ve started from 14.4V few days ago and in the meantime it didn’t have a single load on it at all.
And just only today afternoon, in 2-3 hours after midday, I’ve lost voltage from 13.46V to 13.25V…becase I forgot to turn off the DC isolator. I have no clue what is happening.
Lucky you.
Mines are same panels, same specifications, same brand. Ordered together. And you see, I have problem with them, and he doesn’t.
If the battery was backfeeding into the panels, I would expect to see the solar input voltage at the battery voltage regardless of whether the panels are connected or not. Since the voltage at the MPPT solar input is zero, I would say the MPPT controller is not “leaking” back to the solar input.
I have just turned the DC isolator on to show you. So the connection between the panel and the battery were active when made this pic. It is jumping between 13.10V to 13.13V
Where is the isolator connected? Between the panels and MPPT controller or somewhere else? With the isolator ON, there is battery voltage at the array input which is wrong as the controller state is OFF. Unless the isolator is incorrectly wired, it looks like the MPPT controller is faulty. Can you post a diagram of how the isolator is wired?
Yes, the DC isolator is between the panels and the MPPT.
‘With the isolator ON, there is battery voltage at the array input which is wrong as the controller state is OFF.’
Yes, I saw it. It’s another weird thing.
The isolator wiring is okay I think⬇️
…victron help says that is okay to see ‘state: off’ during nighttime aka in dark. What is now. I mean there is dark outside.
What does the second one mean? That the PV panels voltage must be 5V above battery voltage? The battery is now 13.25V so the PV array needs to be minimum 18.25V to be able to turn on the MPPT?
This is a screen shot of my MPPT controller. You can see that the array voltage is 0.31V (array connected) while the battery voltage is over 52V. This is normal so I would say your MPPT controller has a fault.
The only difference I can see is in your system, you have the MPPT controller earth connection connected to the Lynx distribution box where the earth is connected to the battery negative. I wonder if this might account for the odd voltages you are seeing. If you can easily disconnect the earth wire, it would be interesting to see if that makes a difference.