Q: On my Blackberry I get a "Device Error: 400" and can't get past it. What can I do?
A: This issue is related to the Blackberry device being out of memory. You can solve it by following these steps:

1. Disconnect Blackberry from your computer
2. Open "Desktop Manager"
3. Open "Application Loader"
4. Reset your Blackberry (small hole under the battery door)
5. While the red lamp is on, reconnect your Blackberry to your computer and click "Next"
6. On the following screen you will see either "USB:Unknown" or "USB:", click again on "Next"
7. Now you will see the list with the system applications for your Blackberry
8. Select the applications you need and click again on "Next". The firmware will be reloaded to your device, all data that is on your device will be lost



original link: Device Error 400 - Swiss BlackBerry Forum

Thanks,

My experience has been that some of the units no longer have that little reset function. The hole is there but no switch.
I have searched this forum for a fix to the Device error 400 Restet problem and had no luch so I am going to post what I did to fix it.
1. Disconnect phone from computer.
2. remove battery
3.Turn on BB Desktop Manager
4. Connect Blackberry to USB
5. Run Application Loader with minimal programs checked.
When it is finished you will see error 509.
Now put battery back in and reload the system. When it is complete the hour glass will be seen, maybe for 15 minutes, just wait.

There's always Jl_cmdr if nothing else works.