Battery stopped charging.
It was like cracking open a safe. After I finally managed to pry it apart, my downfall was the battery connector. When I snapped out the old battery tape from the circuit board, the entire connector came off as well. The solder connectors between the connector and the board are extremely fragile apparently, so of course one snapped off inside the connector. After that, there was no hope for repair.
Be very very careful when taking off the battery tape from the connector. On another (successful) battery replacement, I made a little hook out of a paper clip that enabled me to be much more precise when lifting up on the release clip.