Ok, I finally found the chance to get my UPS off the grid and try to charge its batteries with the Multi4 charger, using the new settings you created for me!
I experienced something strange though:
Auto cell detection showed 5 cells. Normal charging preset showed 6 cells, which is the correct. Maybe the charger uses the Charge Voltage to estimate/detect cells?
Anyway, changed cell detection form auto to 6 "cells only" but it wouldn't start.
Changed "Stop Cells Change" to No and charge started.
After about 10 minutes I got Safety CODE 120. In the PC software I saw: "SAFETY CODE: Lithium Cell Cnt not Verified".
See first two attached graphs.
Changed "Stop Cells Change" back to Yes and the charge started OK this time!
After a while the 6 cell count became "VERIFIED". See 2nd pair of attached graphs.
Left it charging for 3h. Seems to have worked fine
The 2nd battery is in a very bad condition. Charge started but soon enough I got cell overvoltage error. It gives very strange readings with other chargers, so it is damaged, the charger couldn't help her
Anyway, thanks for the support, the job was done!