Thank you very much for helping!
I don't know how I can measure voltage directly on the BMS - there seems to be little space for this - see pictures. I tried to dissassemble the 10 small screws
you can see on the metal plate to remove the plate from the circuit board, but the metal plate is glued on and I don't feel comfortable trying to break it off.
By removing the screws I could remove the thin carton plate on the backside and take a photo of the circuit board as you can see.
I measured voltages of the cells again the same way but with the charger connected, and got the following results :
3.35, 3.34, 3.35, 3.35, 3.34, 3.34, 3.35, 3.34, 3.35, 3.35, 3.69, 3.35, 3.35, 3.34, 3.36, 3.34
As you can see, again the cell nr. 11 from left measured considerably higher voltage than the rest.
So what do you suggest i should do?..