Have a 7520 and this was happening. Problem would occur whether phone was connected to USB sync cable or not. Fix (per nextel blackberry support) as follows:
Problem - Due to faulty desktop manager software.
Uninstall current desktop manager. In phone go to your menu item:
Options->Service Book and then delete any references to dekstop (there may be one or two entries).
Download latest BDS v4.0 from:
https://www.blackberry.com/SoftwareD...ient=biTZdMddN
SELECT SP 1 download option!
Install new software. (Select proper email process - if you have BES make sure select when installing option appears).
Once installed, go into BDS interface and then Redirector then choose the Security Tab.
Choose generate key manually and then follow prompts (must move mouse to generate new key until progress bar complete) then apply.
Key will be updated next update (sync)