Dual battery bike not charging in one tray

[tompat]

I have a ebike with Bosch Dual Battery setup (Riese & Müller Supercharger).
Recently the bike stopped charging the battery in the upper tray.

Both batteries work charging stand-alone outside the bike.
Both batteries work charging in the lower tray.

None of the batteries work charging in the upper tray.
The charger clicks a few times rapidly and then shuts off charging if the upper one is the only battery. If both are in the bike it moves on to charge the lower battery.

Funny thing is, it draws power from both battery trays as intended, so it's only the charging that doesn't work in one tray.

Any ideas?

 

[tompat]

Posting this followup if anyone else finds themselves in the same situation.

The bike is now charging both batteries again. I did nothing else than swap the batteries in and out a couple of times. I never take them off regularly.
Guess there was minor corrosion on the battery contacts that wore away when ripping them in and out of the bike.