Time Machine stuck on "Preparing Backup".

Hi,

I open my MacBook Pro, and an error message pops up saying that my backup disk identity has been played with, or someone is trying to use it or its content has been erased.

Hmmm...

I check my Time Capsule, and everything is still there, no problem.

I click on "Back Up Now", and it says it never did a backup before and that this will be the first time.

Since this isn't the case, I reboot my computer.

After rebooting, Time Machine says my last backup was yesterday.

Cool.

I press "Back Up Now", and Time Machine has been stuck on "Preparing Backup" for a loooong time.

I read that I have to remove my "inprogress" file, but I can't find it.

Can someone help me please?

Thanks!

Chris.