I hate to cross post, but I wasn\'t getting any progress with this on serverfault and there appears to be a lot more activity with regard to reporting services here.
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You also can check the Report History, to get this option, put the mouse on top of your report and select manage option from drop-down list of your report, look for the "Report History" on the left options, create a "New Snapshot"
It will show you, the last error know:
Parameter validation failed. It is not possible to provide valid values for all parameters (rsParameterError)
for this case you need to check the definition of the report, I had an issue with the drop-down list I used to send parameters to the report.