Quick update:
When I connected the charger my bms turned on again instantly. I went straight to the app to make some screenshots. When I was done with that I disconnected the charger again to first take look at the pics. Disconnecting the charger turned off the BMS.
It turned out the BMS shut down because it measured one cell to be below my power off voltage of 2.55V. I will attach a screenshot to show you the parameters I set.
But the measurement is not correct. I confirmed with 2 different voltmeters that the cell is, like all the other cells, at 3.35V.
(The BMS app says all cells are at 3.285V, except for #6 which was apparently below 2.55 V. After toroughly cleaning the contacts for balancing cable #6 again and reconnecting the BMS, the BMS turned on normally without the charger connected. On the flipside though it still measures incorrectly, and now displays around 2.85V for cell #6, so basically it still measures the cell to be empty and just barely above the power off voltage.
But as I said, I already measured the cell to be at 3.35V. I also, once again, measured the voltages on the plug side of the cable multiple times, to confirm that there is nothing wrong with the contacts or the wires. I will attach three pictures to show my measurements.
- I measured 20.1V between B- and cell #6
- I measured 3.35V or 3.36V between #5 & #6 and the same for #6  and "off camera" between all other cells as well to be safe
On the main page of the BMS app I got a warning saying "Warning: Equli. Res. too high" with 0.000 Ohm at cell #6.
Another warning says "Sample-wire resistance too large".
Am I correct in the assumption that my cells and the balancing cables and contacts are not the problem, but the problem is somewhere inside the BMS?
If so, I guess I have to get a new one, right? ?
Thanks for reading. If you have some ideas please share them with me ?