Check KB 02942 on BlackBerry support.
According to RIM Support, the folder list on the handheld and the folder list in outlook are not in sync. The sync process occurs every 7 days, so the error should not appear after the next cycle. These are the steps to correct it manually:
1) Turn off "Over the Air Folder Management" (OTAFM) for the user in question by going into the Messages folder, clicking the trackwheel and selecting Options.
2) On the userís workstation, complete cradle synchronization with their BlackBerry handheld (with Reconcile email enabled).
3) With the BlackBerry userís handheld cradled, and within the Desktop Manager software, double click on "Backup and Restore" icon. Click on Advanced. Under the section for the handheld databases, select the entry called "Folders" and select "Clear". Once finished, click Ok to exit out of this portion of the software.
4) Delete PIN folder within the registry at the following location:
HKEY_CURRENT_USER\Software\Research In motion\BlackBerry\PINData\"pinnumber"
5) Delete the following folder within a Windows Explorer:
C:\Documents and Settings\[UserName]\Application Data\Research In Motion\BlackBerry\Intellisync\"pinnumber"
C:\Documents and Settings\[UserName]\Application Data\Research In Motion\BlackBerry\Intellisync\"pinnumber".CFG
6) Turn Over the Air Folder Management back on for this user.
7) Complete another synchronization with their BlackBerry handheld with the Desktop software.