This error occurred while running a report.
The IBM site says:
While running the report, Cognos 8 server will temporarily store all data sets in the Cognos8_install_directory/temp directory. If the disk is out of the space, the error message will be generated.
Assign more disk space on the Cognos 8 server machine.
The other solution is to search in the cognos log files what query is running for the report and look if there are full outer joins in the query.
Perhaps a solution is to make an other selection for the report to avoid the full outer joins.