Thanks to Joost for potentially saving my inverter and introduce me to the concept of forming.
If the capacitors in the inverter are not used for a long time some internal ‘corrosion’ (how I’d call it) is taking place. If you apply full voltage then they can’t handle this due to the thin corrosion layer and they will blow. However if you slowly apply voltage and increase every hour then this ‘corrosion’ proces is reversed and the inverter is ready for use again.
Completed that after I managed to get all contactor box cables in place.