After a second glance at this post another concept comes to mind. With these machines aging as they have we are coming across more CSB's that have failed. If your CSB has failed it can actually discharge one or both batteries instead of charging them. This occurs even while the charge lights show normal steady green.
Here is a quick video which you can use to quickly and simply test that your charger is actually charging the batteries.
If the charger has failed and the battery is allowed to be connected to it for a period of time it can damage the BMS. That could explain the performance issues and the final will no longer start issue. The other posters are very correct as well that it could be a platform fault as well. Good luck troubleshooting!