When you upgraded the ASA from 8.2 to 8.4 did you also update the version of ASDM (put the new ASDM file in flash and update the configuration to point to the new ASDM code)? There is a dependency between the version of ASA code and the version of ASDM code and when you upgrade one you need to upgrade the other also.
HTH
Rick
HTH
Rick