The error code has to do with the sync key of the folder hierarchy. When you sync a number is exchanged between the device and data end. If you make say 10 changes between 10 different sync's than the device will send 10 back. What is happening is the device
is sending back a number that is way off, say 4.
The most common cause of this is resetting the device without adequately flushing the flash file system's RAM cache.
To avoid this in the future, press the suspend button before resetting the device. When the device goes into hibernate mode the cache is flushed. (you may want to wait a few seconds after hitting the button to allow the cache to be written).
With the smartphone, always turn off the phone, and never yank the battery. The ram cache is flushed to flash during the power off process.
Try unselecting mail for sync. Then re-enabling it.