I had the same problem with my 8320 and now with the new 8900. When i try to back up, it fails at content store and it gives me the message backing of database failed. I went through the advanced option and cleared the content store. The value got reduced from 54 to 23 entries. I can manage to clear the 23 entries.
I uninstalled DM 4.5 and upgraded to 4.7. Still the same problem. The 8320 used to give be the same error message but at the browser database.
Does anyone know what can I do to fix this?
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BB Support rule #1: When in doubt, pull it out..... (the battery)
Did you try a hard reset and then try backup again? Other than that, I don't have a clue. (Except also, search the blackberry knowlege base on the exact error message that you are seeing.)