Thank you for this hint! I wouldn’t have otherwise tried to recover with my lab power supply and bought new batteries.
I had left the batteries in the device for a few days (<2 weeks) and some cells were down to below 1V.
I disconnected the battery holders, removed the battery cells and put one battery cell back. I connected the clips of my lab power supply (GOPHERT CPS-3205) to the holder in the right polarity, set it to 2V and 0.25A and waited until current was low (~25mA). That recovered 6 out of 8 cells.
I couldn’t find spec sheets for the delivered batteries, but current values around a tenth of the cell’s capacity should be OK (i.e. 180mA for 1800mAh batteries) based on my later reading.
The other two cells showed voltages below 2V, so I charged them again at 2V and 0.1A and waited until the current showed <15mA, that recovered all cells.
I also made a mistake after the first try of recovery: Since two cells still showed as bad on the battery monitor on the OLED screen, I removed one cell while power was still connected to identify the bad cell on the battery monitor. A few seconds later, magic smoke happened.
After immediate disconnect of everything and initial panic, I found one resistor which is charred (R28). Surprisingly enough it still measures the same resistance values as the other resistors of the same type. I suspected that’s a sense resistor, but after looking at the schematics I’m not sure what it’s for.
I let it charge for a short while and cell voltages looked good, but I won’t use the reform with batteries until I sourced a new resistor and replaced it.