During the installation of the AMmeters I placed a piece of MDF over the cells, and placed my knee on there to keep balance.
It wasn't my full weight, but apparently too much.
The threaded rod of the affected cell was slightly higher then the rest (5mm)
Here I'm happy to even find threaded rods with "allen wrench" (hex) , I can't be choosy.
Pressure on the thread/terminal apparently pressed out of place, and created a short inside the cell.
That got too hot and set the MDF sheet on fire, smoldering as MDF doesn't really burn well.
My walls and ceiling are covered with concrete fiber sheet, to contain accidents, but I doubt they can hold back fire long...
I guess I can say I got lucky, that it didn't burn our house down.
That would be a real shame.