New "backup failure" problem

I'm running OS 4.6.0.282 and DM 4.6.0.19 on a Bold

I've had repeated problems backing up. Originally cleaning out the cache worked.

Now when I try to back up, the program chugs along ok until it hits calendar where it dies and I get the super informative message:

Error: Unknown error

I'm feeling exposed without the ability to do a back up. Has anyone seen this error before? Has anyone solved the problem?

I know that a back up will succeed if I use DM 5.0, however my corporate IT only supports 4.6, and 5.0 won't work with our archaic email system.

Thanks for your help.