Welcome to the forum.
There are several threads where others have posted about having this same sort of issue. Maybe some of this will help you.
Since you have canion, How much difference was there between the max and min cells, and how low is the lowest cell when this occurs?
The first thing to check is the age and condition of your 12-Volt Aux battery (the lead acid starter battery). Sometimes folks can get an unusually long "life" out of these batteries, but when they get old, weak or worn, they can cause this error.
The fact that charging appears to solve the issue temporarily indicates to me that your aux battery may be weak. During chargging the DCDC converter will try to chargge up the aux, so for a brief time it has some extra boost. But it is short-lived in a weak battery and the warning light returns. If you have the original OEM aux battery, then i'm fairly certain that is the source of your issue.
It could be that you have a bad cell, but you should be able to see that with canion. Does it chargge back up to full and show 16 bars on the gauge. Is one or more cells lagging all the rest when full?
There are other issues that can cause the HV warning (! on car), but you would need to get a reading of the Diagnostic Trouble Code, DTC, to troubleshoot the problem. the dealer has a tool to read these, but there are also aftermarket tools that can do this for the miev. i use the iCarsoft model i909 to read these codes, there may be other brands and versions online.
Good Luck and let us know what you find.
kiev = kenny's innovative electric vehicle