feureau, if factory reset doesn't work. Try re-flashing the official firmware update (again) even if you already are on that version.
http://www.canon-europe.com/Support/Consumer_Products/products/cameras/Digital_SLR/EOS_M.aspx?type=firmware
http://www.canon-europe.com/Support/Consumer_Products/products/cameras/Digital_SLR/EOS_M.aspx?type=firmware