Really does sound like a deep discharge problem. Whether this is because the charging behaviour changed with the update is another question. I don’t recall the FP2 being able to start without a battery present, so the crash during the app optimisation is likely due to the phone running out of power. The power draw during that stage is quite significant, and probably exceeds the rate at which the phone can charge. This is also why when you run the updater app, it refuses to update unless the phone is charged for >80%. Hence @paulakreuzer’s comment to wait until the light turns green - you’ll need a high enough battery charge to get the phone fully started.
There’s a more in-depth of the deep discharge problem in the topic below. As your phone needs to be able to finish app-optimisations you should definitively wait more than the 10-15 minutes indicated in the guide (rather wait for the light to become green). Try your collection of chargers and cables if you can’t get a steady charging light with first combination you try. If nothing works to charge the battery, contact support as there may be a problem with either the battery or the charging connector.