There are many reports on the internet of people encountering Err 70 under a wide variety of circumstances. It happened once to me too.
I have been using my own 5D Mk II for years with GBTimelapseEOS with no problems.
In case you do get the Err 70, here is the reset procedure recommended by Canon...
- attach lens
- remove main battery
- remove cf card
- remove datetime battery and tray
- turn on
- wait 30 seconds and turn off
- wait at least 30 minutes
- replace datetime battery
- replace main battery
- replace cf card
Here's another sequence to try
- Power on camera
- Select C1/C2/C3 - these modes work even with Err 70 (but are kind-of useless as you can't assign anything to them)
- In the menu, reset as many settings as possible to default
- Power off the camera
- Remove the lens (and possibly CF card, although I didn't)
- Select CA on the dial
- Power on again - now you should get a minimal menu
- Switch away from CA to another mode