I am getting the following error message for every job when submitting via cassub (after the initial attempt which was successful):

110913 20554755       5952    CASKC0027E Error executing service 'CASA_Sub_JCL'
Load error : file 'MFJXXIT'  error code: 173, pc=0, call=1, seg=0
173     Called program file not found in drive/directory 20:55:47

A possible reason for this is that "program_search_order" is set to "3" (either in the region's Configuration Information: [ES-CONFIGURATION] section, or in the COBCONFIG_ file).
The program_search_order tuneable is a global setting and so will impact both the system ES code as well as the user application code (since they both run within a container in the ES cassi process).

Setting this to 3 means that one of the paths that is NOT checked is the calling program's path and since this affects system code (as well as user code), the 'program not found' error results for MFJXXIT.

Setting program_search_order to any other number other than 3 (i.e. 1,2 or 4) will include the "calling program's path" in the search order so you won't hit this error.  The default value for this setting is 4.