Quite often (every 10 minutes) my multiplus II decides to reject the AC-in (grid) for about 1 minute in a dramatic way, firing off all kinds of relais sounds (see video). Where can i find the reason of this decission and fix the problem? It gets AC-in from a Voltage stabalizer so should be fairly stable 
I would at first remove the “voltage stabilizer” and see if the Issue remains.
You could also review VRM Advanced graphs for the multis ACIN Voltage / Frequency, if something appears there. Mind, VRM graphs are 1minute average values, so seeing millisecond-peaks may be impossible or look way lower than it actually was.
There is some more info on the disconnect reasons on dbus, see Need official mapping for VE.Bus D-Bus - #4 by cmock
Hmm yeah, I’m seeing 300+ Volt peaks in that graph. Especially when under higher load (electric car charging + well pump engaging). I guess it’s a good thing then that my Victron protects my gear from a flakky voltage stabalizer.
I can’t remove the voltage stabalizer as it will bring Voltage down to even 150V under heavy load.
Sounds like you have bad neutral connection.
What’s a bad neutral connection?
Check your wiring from your incoming.
The low voltage is just from the grid. I’m in Portugal and e-redes refuses to fix it as I’m far away on a mountain. Nothing to do with a bad connection.
Ah right. So a poor grid supply.
AVRs depending on their regulation type cause issues with the LOM on the inverters.
In some areas that have a long line to supply such as yours we actually have a 2 inverter system.
One that inverted all the time (no connection to grid) and the other in charger only mode to charge from the grid as needed.
When the grid is poor and the avrs works hard you will find it is consumed.
Is your system a split phase or just 2 of 3 phases?
You could disconnect your house from the grid. The MP2 or Multi RS will create an island/micro grid.
Than you could use a very reliant telecommunications PSU to permanently recharge your battery/ DC bus.
With an MP2 you might loos 10-15% energy from grid with a Multi RS less. PSUs convert at 99%.
Professional systems do the same AC-DC-AC conversion with high efficient high frequency inverters but costs thousands of EUR.
Suitable 2-3 kW PSUs might be Huawei and Eltek Flatpack 2 which are available at 100-150 EUR at eBay.
Would this work if I also want to start feeding back into the grid my solar surplus
No. But really you don’t want to be doing that through an AVR and definitely not into an unstable grid with voltage drop like you are experiencing
I was planning to move the avr behind the victron and limit the grid use so voltage drop won’t be as big.
Oh and I think also a well pump with high starting current is contributing to the disconnect issue as the avr doesn’t have enough time to react.
You could use a single MP2 or Multi RS 230 to connect to the grid and recharge battery and feed back to grid.
With that solution your house is still independent with stable voltage, but the single MP2 / RS might disconnect from grid as it’s loss of mains detection (LOM) might not accept your unstable grid.
Additionally the MP2 will only feed back ~80% of your energy due to losses and standby consumption.
Short a telco PSU will accept your grid and charge battery highly efficient but not feed back. It’s reliable, cheap (used) but loud. Integration is via ESP32 and open-source.
An MP2 can be “natively” integrated, charge and feed back but is less efficient and might drop you grid regularly.
You need to do the math, how much you loose on not feeding back into grid.