VIV-SC-4200 battery charger - what do multiple blinking circles mean?

I have a Vivitar "Universal Battery Charger", and the model number (VIV-SC-4200) is only found in small print in one place. The manual says "As your battery is charging, the LCD display screen on your charger will display a series of circles (25%, 50%, 75% and 100%) to show the progress of your charge." And "When your battery is fully charged, the LCD display screen on your charger will show four circles to indicate your battery is 100% charged."

No idea what it means when the 25% and 100% are both blinking. Does it mean one of the two charging batteries is nearly full and the other nearly empty?

As I've tried leaving the batteries (two AA size NIMH 1.2V, chosen out of either 4 old proprietary 1500 mAH or 4 new name-brand 2000 mAH) in it overnight, but then they get really hot, with no change to the indicators, I'm still baffled.

And since the manual says "You must have two AA or AAA batteries charging inserted in your charger at once in order to charge.", I'm thinking inserting only 1 at a time would be a bad idea.

UPDATE: Got a response back from Sakar International, who apparently handle customer support for Vivitar or something like that, and they suggest trying only one battery at a time. Although that's clearly contraindicated in the manual, I did try it. The device won't even recognize that there's a battery inserted, on either of the two sides, and stays at the display state that indicates it's trying to determine if it's a LiOn or NiCD or NiMH type. So still stuck... Luckily, I don't have to use the charger much now, as the device that I was trying to keep charged that used them is no longer needed.