Thanks, this is what I was thinking the whole time too. Unfortunately the "house battery" switch went wrong and I could not start the engine - until I found out that pushing the switch key helps, I was lucky that I was not alone.. Switched off the first switch (the motor switch?) but everything is still working with the "house battery" switched on. I am the second owner, so maybe someone made some "undocumented" changes or connections, II will have to follow the 95sqmm wire coming off the first switch to find it out I think.
I replaced the original Quick QIS15N with max. 150A permanent load with an Osculati Heavy Duty MKII 14.385.10. (280A max permanent load) The new one needs a larger hole (28mm instead of 25mm) but fortunately the original hole was big enough.