I seem to recall reading somewhere that this is an issue with DTM 4.6, maybe try and get your hands on 4.5 and see if that helps. I use 4.5 with no issues, but at the same time, I have not tried 4.6.
Otherwise give us some more info ............ are you on BES or BIS. The wireless sync and backup indicates to me BES and therefore may mean an IT Policy is in place that would prevent you from wiping the device; not that you're trying to wipe it. However an IT Policy could be at play.
If not currently on BES, did you buy the blackberry new or used? Used BB's could have an IT Policy installed that was not removed prior to sale.
I don't believe the quantity of messages would cause a failure.
Welcome to BBF by the way!