IBM Content Search Services (CSS) getting FNRCB0035E: CBR_TS_ACCESSING_INDEX_FAILED while indexing.

Symptom

Indexing no longer works and the following error is logged to the p8_server_error.log

2012-06-26T18:02:40.453Z 688F688F CBR FNRCB0035E - ERROR Execution of index requests failed.
com.filenet.api.exception.EngineRuntimeException: FNRCB0035E: CBR_TS_ACCESSING_INDEX_FAILED: An IBM Content Search Services server could not access the null index or the directory that contains the index. The attempt to delete the index failed for the server that runs on the <Server Name>:8191 host and port. Verify that the file permissions for the index file and directory permit access to the server operating system user account. Also verify that the index directory is accessible to the server across the network. If this error reoccurs after you fix the problem, or if the index file does not exist, call IBM Software Support.

FNRCB0041E: CBR_TS_QUERY_FAILED_EXCEPTION: An IBM Content Search Services server could not run a full-text search. See the p8_server_error.log file for the error message stack that includes this error for more information on the specific cause of the error. For information about the location of this log, see "Viewing the IBM FileNet P8 log files" in the IBM FileNet P8 information center. If you cannot determine the cause of the error, restart the server. If the error continues to occur, contact IBM Software Support.

Caused by: com.ibm.ecmts.client.api.GeneralClientException: IQQG0037W The collection /tmp/OS1_Document_20120430145554_25B07734249D4AC0A9125AF8F2129F30 does not exist.
Causes of the problem:
IQQG0020E com.ibm.siapi.SiapiException: Message0: SEVERITY_ERROR: Message ID: [] Resource Bundle: [] Message Text: [Failed to obtain searchable]
IQQG0020E com.ibm.siapi.SiapiException: Message0: SEVERITY_ERROR: Message ID: [] Resource Bundle: [] Message Text: [Failed to obtain searchable]
IQQG0031E The object with name /tmp/OS1_Document_20120430145554_25B07734249D4AC0A9125AF8F2129F30 cannot be loaded into cache <Searchable Cache>.
IQQS0003E The searchable object for collection /tmp/OS1_Document_20120430145554_25B07734249D4AC0A9125AF8F2129F30 cannot be loaded.