Te encontré esto, es de otra Canon pero... el hard reset le solucionó el error 20:
I've seen it once on my 5D II. A hard reset worked for me (first 5 steps below), else I would have had it repaired.
Quote from Canon Support:
It is possible that this error is related to the CF card being used in your camera. We recommend that you try formatting the CF card by following these steps:
<< cut obvious formatting manual >>
If the issue persists, please try an alternate CF card in the camera. If the alternate CF card works fine, then your first CF card was the cause of the issue. If not, there may be another issue with the camera. If so, please follow these steps:
1. Turn off the camera.
2. Remove the lens, battery, and CF card.
3. Allow the camera to sit without power for approximately 20 minutes.
4. Insert a fully charged battery, and turn on the camera.
5. Depress the shutter button as you would to take a picture.
If the message does appear, your camera may require service.
If not, please complete the following steps:
1. Turn off the camera.
2. Clean the lens contacts by gently rubbing them with a pencil eraser or soft cloth. Be careful that you do not let any debris fall into the camera body.
3. Reattach your lens.
4. Turn on the camera.
5. Depress the shutter button as you would to take a picture.
If the message still appears, your camera will require service.
Un saludo.
