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.