Try booting into Safe Mode according to How to use safe mode on your Mac and try to complete a backup from there. If you're successful in Safe Mode reboot normally and try another backup.
Note: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.
If you're still unsuccessful you can do what I had to a while back when TM couldn't get beyond preparing: erase the TM drive and start over from scratch. This is a good method if you don't have any files on the TM drive that are not on your boot drive that you need.