1. The SQL for the output is missing an outer join (or has other sql issues) .
2. The TEMP tablespace on the Oracle database, used to temporarily store query data, has filled up and cannot hold further data. (In these situations, a DBA will either increase the tablespace or take other steps to resolve the issue).
If the log indicates no errors, then the issue is likely a problem with the output SQL. You can view, recompile and test the SQL in the User-Defined Output Entry screen. A TEMP space issue will write a message to the log after the job has failed, though it may get stuck in a holding pattern awaiting more temp space to be released.