Canon admits Version 2.0.3 firmware for the Canon EOS 5D Mark II causes a malfunction in which the manual recording level changed and made recording impossible. The correction is a new release Version 2.0.4 which is available from the Canon web site at:

http://web.canon.jp/imaging/eosd/firm-e/eos5dmk2/firmware.html

Thanks to Adam Forgione for sharing this information