ERR 20 is supposed to indicate a mechanical fault, particularly the mirror, it's operating mechanisms and motor(s)... (ERR 30 is usually shutter related). But it also might be something as simple as too low power, power interruptions, or even garbled firmware "instructions". The mirror and shutter cycling of a camera is very "clock like", high precision mechanisms going through a carefully timed sequence of events. It's a pretty reliable mechanism, but things do wear or just "go off the tracks" and foul up.
Yes, if you have a battery grip on the camera, check that it's tight. It also might be a good idea to remove it and clean the contacts, both on the grip and up inside the camera.
It would be a good idea to also clean the contacts between battery and grip (or between battery & camera, if not using a grip).
Probably wouldn't hurt to also clean the lens to camera contacts.
In all cases, I recommend using a clean cloth rag moistened with few drops of isopropyl (rubbing) alcohol, and wiping the contacts with that. Just be careful to keep off the optics and mirror. And don't get things too wet.... You just need to moisten the cloth a little.
To get to some of the contacts and wipe them, you will probably need a tool to wrap the cleaning cloth over. Those wooden coffee stir sticks that Starbucks provides its customers are pretty good, but anything similar made of wood or plastic might do. It's just a matter of getting some extra reach to gently wipe the contacts up inside the camera and/or down inside the battery grip.
Leave things apart to air dry for ten, fifteen minutes or so. Isopropyl alcohol evaporates pretty quickly (check to see if the cloth you used has dried out... If it has, then most likely the contacts you cleaned with it are dry too).
Another thing... Inspect carefully the leading (bottom with the camera upright) edge of the mirror... If you see any gooey black residue that might indicate the foam light seals are going bad. Sometimes they get sticky and that can lead to the mirror hanging up, messing up the timing of the mirror/shutter cycle. Usually the foam light seals are good for 4 or 5 years, but they can go bad faster if the camera is often used in hot environments, for example. The foam seals are glued in place and, when going bad, that glue can transfer to the edges of the mirror... That's what you are looking for. If you see any, a local camera repair technician probably can help you. Light seal replacements aren't a big deal (5DII only uses them along one side of the mirror), but not something I'd recommend an inexperienced person tackle because the mirror is right below and the focus screen just above, both pretty easily damaged.
Also you might want to do a "reset" or "reboot". To do that quickly, try the following:
1. Turn camera off.
2. Remove both the main (rechargeable) battery(ies) and the small silver memory battery. You'll need a precision cross-point screwdriver to get to memory battery. In the 5DII it's on the lefthand side, partially covered by the rubber socket covers, near the bottom. Remove the screw, then slide out the battery.
3. With all the batteries out, turn the camera back on.
4. Press the shutter release button once. The camera won't fire, but this will drain all the remaining power in the camera.
5. Turn off the camera.
6. Reinstall the batteries.
7. Turn on camera, go into the menu. Check the time/date. It should need to be reset, if you got a proper "reboot" of the camera.
8. If not, repeat the process steps 1 through 7. If you wish, instead of turning the camera on and pressing the shutter release, you can just turn it on and let it sit for an hour or more. That also should fully drain any remaining power. Then repeat steps 5 through 7.
9. If you can't get the camera to "reboot" properly, try leaving the batteries out an even longer time, such as overnight. If you still can't get it to "reboot", you may have to send it to Canon.
The camera should be able to shoot without the memory battery, but this might be a good time while you have it out to replace that battery. They normally should be good for 4 or 5 years, but who knows? If you wish, you can take the memory battery some place like Radio Shack and have them test it... But it's only a few dollars for a replacement and they're pretty widely available, so might be easier to just go ahead and replace it. Be sure to wipe off finger oils before reinstalling it, and to install it in the correct orientation (it should only fit one way).
If cleaning the contacts and doing a reboot don't stop the camera from throwing off ERR 20, the next thing you can try is reinstalling or updating the firmware. Sometimes it seems as if the firmware gets muddled and starts to cause problems. You can get instructions on updating/reinstalling the firmware and download a fresh copy from the Canon website.
All the above are a bit long shots... might help, might not. But all are pretty simple to try, cost little or nothing, and might work. I always figure they're worth a try, before going to the trouble of sending the camera in to Canon service.
