App Error 523 is a software hiccup usually caused by a 3rd-party application that is not compatible with the BlackBerry device OS.
Follow the below steps to get rid of this error message:
1. Take the battery out and put it back in after 30 seconds or so
2. While the device is rebooting (or while the hourglass is still spinning), plug it into the PC
3. Then open the application loader and remove the 3rd-party application that’s causing the issue
4. Worst-case is when you need to wipe the device using JL_Cmder. Follow the instructions here: How do I wipe the BlackBerry using Jl Cmder?
Thanks to John Clark from BlackBerryForums for the tip!