Reform turns off at 60-70% of charge

I am experiencing the weird issue.

After discharging from 100% to around 60-70% one of cells (in exact place of battery block) suddenly drops voltage from 2.9-3.1 to 2.0-2.2 and Reform turns off. I thought the problem was caused by a defective cell, but after changing that exact cell to another one, charging, and discharging I got the same problem – new cell in the same place dropped it voltage and Reform turned off.

What it could be? Defective battery board? Software? Cells?

Are there any ideas?