The Cognos server was out of disk space because cache files keep filling the folder:

C:\Program Files\ibm\cognos\analytics\data\datafiles\datasetCache

This happens because multiple datasets were defined for Cognos reports. Every time the dataset was refreshed, a large cache file was created but the cache file was never deleted, even after the Cognos service was restarted.

The default age of dataset cache files is 7 days (604800 sec), but as a more practical setting you can set it to half (302400 secs).

The setting is located in:

C:\Program Files\ibm\cognos\analytics\configuration\metadata\xml\metadata_DatasetService.xml

Locate the following entry:

< parameter name="maxAge" type="long" label="Maximum age of unused dataset cache files (seconds)" default="604800"/>

You must restart the Cognos service for the setting to take effect.