I have the same issue as well. I get the error message when backing up the messages database as well as the browser data cache database. What I did was click on Advanced and backed up everything except for what was giving me an error message. Hope that helps.
I had the same problem yesterday. My culprit was a corrupted browser cache.
Standard things to try first: battery pull, restart DM.
After that if you still have the problem I would recommend using the advanced backup screen to manually backup each file one at a time. At some point, you should encounter the problematic database. Depending on which it is, you could decide to clear it (and/or reload a previously working copy of that database).
