First, see this: What should I do with App Error 523? - BlackBerryFAQ
"523" is an error code, not an app. It usually means something you need to reload your OS. AppLoader.exe is included in Desktop Manager (Windows only) and is accessed via the command line.
1. Backup all your data if possible. Sometimes a corrupted OS won't allow this, so hopefully you have a recent backup. The wipe/reload process wipes your BlackBerry clean.
2. Wipe with JL_Commander: How do I wipe the BlackBerry using Jl Cmder? - BlackBerryFAQ
3. Download the latest version of the OS for your device. There's a link at the top of the 8500 Forum for latest OS builds for each model. Make sure you download the OS for your exact model. That means NOT downloading the 853
0 OS for your 852
4. Make sure you have the latest version of Desktop Manager: BlackBerry - BlackBerry Desktop Software
5. Install Desktop Manager, then install the OS you downloaded for your 8520.
6. Update the OS on your wiped 8520, making sure to follow the instructions in the BBF FAQ: How do I upgrade the OS on my BlackBerry? - BlackBerryFAQ
Make sure you delete the vendor.xml file as per the instructions. Failing to do this will prevent the update process from running. I've forgotten this step and was on the verge of pulling out my hair trying to figure out what was wrong. Just look at my profile/avatar pic!
7. When you reload your third-party apps, go slow. Usually this Error 523 is caused by an app that wasn't written properly, conflicts with another app, or just gags your BlackBerry.