Does it happen on both the imac and the MBP?
Does it happen at the same time (assuming yes to the above)?
Have you done any upgrades to OS, since installing the TC or were both computers running Lion when this happened?
Do you store anything else on the TC or is it purely used for TM backups?
Did you talk to Apple support people about it? I think it is important for you to talk to them so the issue is being recorded, even if they don't have a solution. The problem is reported fairly often here.. but none that I see have reported it happening on a regular basis.
See pondini KB
http://pondini.org/TM/C13.html
I would be inclined to do as Pondini says, but include a disk erase and perhaps even a reset of TM. Do the setup of the TC again and make sure you run the first backup by ethernet. That might mean a bit of work to actually rearrange your network back to previous setup before you introduced the TC.. Turn off TM on one computer so you can plug the other in directly by ethernet.. run the backup.. it will be fairly quick by ethernet cf. wireless. Once finished swap it over. I would also be inclined to do a full backup image of the computer using another software.. CCC for instance. That means you have a backup which is not continually written to and removes the possiblity of corruption.
Keep track of what you do.. and if the failure happens again, which unfortunately it well might.. I would look to avoiding using wireless or go to some effort to setup wireless to use 5ghz on the laptop and perhaps cable the imac.. Long backups over wireless if you are in a situation where you get interference could be the issue.
