2021 GTI SE suddenly says wrong key and display flashes

[soccerdad]

I stopped in the middle of a ride to take a picture. I left the key on the post and killed it with the stop button. When I went to start it less than a minute later, it sounded like it was about to crank and then the display reads something like the key is not correct. And the display backlight started flashing and components on the motor, like the throttle body were clicking on and off in random patterns. This all would never stop and I had to disconnect the battery. It starts all of the flashing and clicking again as soon as I connect the battery and hit the start button. It is a 2021 GTI SE 170. Any ideas on what it could be or how I can start to trouble shoot it? I am guessing a bad connection or a bad ECU but that is a guess. Any educated guesses or input are welcome! I don't even know where the ECU is on this thing. I had it in for warranty service a year ago for a freaking out gas gauge and they replaced the main harness. That fixed it, but maybe they left something loose?

 

[skiasylum]

Check all your grounds for corrosion or a bad connection.

 

[soccerdad]

It was the battery. I towed it back and eventually swapped the battery out with my other ski. Bam, started right up. But the odd thing is that the battery tests at 12.6VDC. But on a charger it immediately goes into trickle mode because the charger thinks it is full. But then put the battery in my other ski and it had the same no start, just a click problem. So the battery had just suddenly died with good voltage but NO amps. Odd. Maybe a bad batch of batteries in 2021 (covid times...)