you would need to wrap double quotes around the entire path:
$L_ETL_FILENAME = '"' || GET_ENV ('DS_COMMON_DIR') || '\LOG\[$L_ETL_EXEC_FILE]"' ;
if you do a directory listing of just the path... print(exec('cmd','dir "' || GET_ENV ('DS_COMMON_DIR') || '\*.*"',8)); ... it should tell you if thats the problem.
I know that I had issues when they changed the install directory, because they decided to add blanks in the path (for whatever reason) and when you print the stdout its not entirely clear if theres a blank in there as it spans multiple lines in the log. I dont have direct access to the job server so its faster for me to do a dir *.guess than it is to get an answer out of the admin. ;(