So I'm reviving this thread of mine, because my post #2 was incorrect. Still experiencing the same original issue. Basically, after about 2-3 days, Time Machine backups just stop. Nothing interesting in the logs. Everything is backing up correctly, and then backups simply stop. This is happening on a Mac mini that is running 24/7 as a media server, so it does not get 'regular' restarts. When TM stops backing up, I'll restart the computer and then it'll work again for a few days, then again, just stop. Note, this is not the issue some have with it getting stuck at 'preparing backup'. I'll also add that Manual backups work correctly. It's like the chron job just poops out for some reason.
My current workaround is to use a calendar automator shell script to force Time Machine to backup once a day, so that I at least get a daily backup.
Is anyone else having this issue?