IWR prompt report gives the following error message: Unable to execute report.

Symptom

browser error message:

The report request failed.
The execution of this report at 10/24/2007 08:30:58 failed due to the following reason:
Unable to execute report.;

iwr.log:

1. File Name: \bi\iwr\7.4\mr1hot\win32\src\iwr\iwr\dispatchers\irsdispatchers\irsinstance.cpp; Line#:513;
Desc.:IRSInstance::executeRunReport;
IWRcer5:IWIRSDISP:ErrorHandlerError: SWID=20071024083048418KLNSRV02PID10280TID103603240 - KLNSRV01,
8136(138), 2007/10/24 08:30:59.004; Error: 0, Generic;
Process ID: 8136; Thread: 312; Host: KLNSRV01; Text: Execution of the job <reportname> failed. Reason:
Unable to execute report
\\xxx\<reportname>.imr. Reason: Error: 0, Generic;
Process ID: 7224; Thread: 7624; Host: KLNSRV01; Text: This report needs prompt information;
BackTrace Info:
1. File Name: .\app.exe\ole.lib\appauto.cpp; Line#:8189; Desc.:This report needs prompt information;
2. File Name: .\app.exe\ole.lib\appauto.cpp; Line#:8773; Desc.:CAppAuto::ExecuteRunReport;
...
IWRcer5:IWReqMan:ErrorHandlerError: SWID=20071024083102270KLNSRV01PID7888TID73325261 - KLNSRV01,
6580(130c), 2007/10/24 08:31:02.395; Error: 0, Generic;
Process ID: 6580; Thread: 4876; Host: KLNSRV01; Text: The following error was sent to the Consumer. Error number: 11 .
Error message:
The report request failed.<BR>The execution of this report at 10/24/2007 08:30:58 failed due to the following
reason:<BR>Unable to execute report.;
BackTrace Info:
1. File Name: .\iwrreportcmd.cpp; Line#:646; Desc.:The following error was sent to the Consumer. Error number: 11 . Error
message:
The report request failed.<BR>The execution of this report at 10/24/2007 08:30:58 failed due to the following
reason:<BR>Unable to execute report.;