Hi Sebastian,
You can keep breakpoint in Function module 'CRM_MKTLIST_STAGING_UPLD_TO_AP' at line
CALL FUNCTION 'FILE_GET_NAME_USING_PATH'
and retrigger ELM process, then while debugging please check corresponding values for variables.
GC_PATH = MARKETING_FILES
LV_FILE_NAME_WITH_PATH =
Verify the variable "LV_FILE_NAME_WITH_PATH" path name. This is actual physical path to be maintained in transaction FILE.
Hope this helps you.
Thanks and Regards,
Anup