Time Machine stuck in "Preparing Backup..."

Hi, my time machine backup on my ReadyNAS NV has stopped working recently. Whenever I try to kick off a time machine backup, it will go into the "Preparing Backup..." mode, and get stuck there. It never gets out of that mode even after many many hours (10 or more). Can someone help me figure out what the problem is, and how to fix it?

Thanks in advance.