How can I find the cause for "invalid export data"?

Hi,

I have a business application using Crystal Reports Server 2008 SP5 as back end for report creation, using the web service API (Session and ReportEngine service). Multiple users can request documents in parallel, so the business application uses multiple sessions at one time. In times of higher demand, I encounter more and more errors of type "invalid export data" returned by the web service API.

How can I find the cause for these errors? In the logging folder of the Crystal Reports Server I cannot find any useful information up to now.

Thanks for hints

Joern