Options -> Security Options -> Key Store, Allow Key Store Backup/Restore set it to No
Then try to back it up see if that helps.
If not Try:
Instead of going into Backup go into Advanced and backup your more important databases, or try backing them up one by one till you find the one giving you the problem.
8830 -> 8330 -> 9550 -> 9650
Just think about how far BlackBerries have come from then till now... And what else is coming.
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