An Unexpected Error Occured In Report Processing. (rsInternalError)

Hi all,
Our application that uses Reporting Services for it's Reports.
On one customers build it appears to keep falling over.
Restarting the server seems to resolve the problem for a short period
and then it happens again.
The exception that is returned to our application is
"An unexpected error occured in report processing. (rsInternalError)"

The customer has sent me the log files from
C:Program FilesMicrosoft SQL ServerMSSQL.1MSSQLDataReporting ServicesLogs

I've tied the time that the issue occurs to this section
w3wp!chunks!1!04/11/2008-10:27:35:: e ERROR: GetChunkPointerAndLength: System.Data.SqlClient.SqlException: Database 'ReportServerTempDB' cannot be opened due to inaccessible files or insufficient memory or disk space. See the SQL Server errorlog for details.

and

w3wp!processing!1!04/11/2008-10:27:35:: e ERROR: Throwing Microsoft.ReportingServices.ReportProcessing.ReportProcessingException: An unexpected error occurred in Report Processing., ;
Info: Microsoft.ReportingServices.ReportProcessing.ReportProcessingException: An unexpected error occurred in Report Processing. ---> System.Data.SqlClient.SqlException: Database 'ReportServerTempDB' cannot be opened due to inaccessible files or insufficient memory or disk space. See the SQL Server errorlog for details.



I doubt that the disk is full as our database is sitting on the same drive and presumably the whole system would have stopped working by now.

Any thoughts as to what the problem might be?
We've not seen this before.

Cheers,
Chris