SSRS WebService ReportExecution2005.asmx raised an exception during creating pdf

Question

Hello,
yesterday i created already a task regarding this problem, but i think it was not detailed enough. Since yesterday i tried a lot of things and came to the following result.
If we call the WebService ReportExecution2005.asmx with format "PDF" like
result = this.ReportingService.Render(
"PDF",
null,
out extension,
out mimeType,
out encoding,
out warnings,
out streamIds);
we get the following exception 
System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.Security.SecurityException: Request for the permission of type 'System.Security.Permissions.SecurityPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.
at Microsoft.ReportingServices.Library.StreamFactoryBase.CloseAllStreams(IEnumerable`1 streams)
at Microsoft.ReportingServices.Library.StreamFactoryBase.CloseRegisteredStreams()
at Microsoft.ReportingServices.Library.StreamFactoryBase.Dispose()
at Microsoft.ReportingServices.Library.WorkspaceManager`1.Pop()
at Microsoft.ReportingServices.WebServer.ReportExecution2005Impl.Render(String Format, String DeviceInfo, PageCountMode pageCountMode, Byte[]& Result, String& Extension, String& MimeType, String& Encoding, Warning[]& Warnings, String[]& StreamIds)
at Microsoft.ReportingServices.WebServer.ReportExecutionService.Render(String Format, String DeviceInfo, Byte[]& Result, String& Extension, String& MimeType, String& Encoding, Warning[]& Warnings, String[]& StreamIds)
The action that failed was:
Demand
The type of the first permission that failed was:
System.Security.Permissions.SecurityPermission
The first permission that failed was:
<IPermission class="System.Security.Permissions.SecurityPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
version="1"
Flags="ControlPrincipal"/>
The demand was for:
<PermissionSet class="System.Security.PermissionSet"
version="1">
<IPermission class="System.Security.Permissions.SecurityPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
version="1"
Flags="ControlPrincipal"/>
</PermissionSet>
The granted set of the failing assembly was:
<PermissionSet class="System.Security.PermissionSet"
version="1">
<IPermission class="System.Security.Permissions.SecurityPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
version="1"
Flags="Execution"/>
<IPermission class="System.Security.Permissions.StrongNameIdentityPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
version="1"
PublicKeyBlob="0024000004800000940000000602000000240000525341310004000001000100512C8E872E28569E733BCB123794DAB55111A0570B3B3D4DE3794153DEA5EFB7C3FEA9F2D8236CFF320C4FD0EAD5F677880BF6C181F296C751C5F6E65B04D3834C02F792FEE0FE452915D44AFE74A0C27E0D8E4B8D04EC52A8E281E01FF47E7D694E6C7275A09AFCBFD8CC82705A06B20FD6EF61EBBA6873E29C8C0F2CAEDDA2"
Name="expression_host_de60dfd5adf64cdfa1c847505f88a637"
AssemblyVersion="1.0.0.0"/>
<IPermission class="System.Security.Permissions.ZoneIdentityPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
version="1"
Zone="MyComputer"/>
</PermissionSet>
--- End of inner exception stack trace ---
which is really strange. This error depends on the content of the report. But the following is really strange. If we do the following steps (First call with format "WORD" and then with "PDF", all is fine. So it seems that the initialization
seems not ok. It seems that the WORD format call initialized something which allows to process the PDF conversion.
result = this.ReportingService.Render(
"WORD",
null,
out extension,
out mimeType,
out encoding,
out warnings,
out streamIds);
result = this.ReportingService.Render(
"PDF",
null,
out extension,
out mimeType,
out encoding,
out warnings,
out streamIds);
When we use the Web ReportViewer we can open and export each report without any problems. So it seems that something is different between the ReportViewer and the WebService. I already checked also the ServerReport class from the WebForms.dll, but the result
is the same. At first i have to render in another format and the into the PDF; otherwise i don't get it to run.
 So maybe somebody know this problem and give me an answer.
best regards
UPDATE :
I already checked it also on a 2012 Server (SQL Server and Reporting Server), because the previous check was running on a SQL 2008 Server. The result of the both tests is the same, but i will get another Message from the SQL Server 2012. The message means
that the Field 'ShowWarehouseOwner' does not exist in the dataset, which is not right. It exists. When i call the report the report at first with 'WORD' and then with 'PDF' all is fine.
Find attached exception from the Reporting Server Log
processing!ReportServer_0-2!105c!10/24/2014-14:19:01:: e ERROR: Throwing Microsoft.ReportingServices.ReportProcessing.ReportProcessingException: , Microsoft.ReportingServices.ReportProcessing.ReportProcessingException: Exception of type 'Microsoft.ReportingServices.ReportProcessing.ReportProcessingException' was thrown.;
library!ReportServer_0-2!105c!10/24/2014-14:19:01:: w WARN: Microsoft.ReportingServices.Diagnostics.Utilities.RSException: The Hidden expression for the tablix ‘Tablix5’ contains an error: The expression references the field 'ShowWarehouseOwner', which does not exist in the Fields collection. Expressions can only refer to fields within the current dataset scope or, if inside an aggregate, the specified dataset scope. Letters in the names of fields must use the correct case. ---> Microsoft.ReportingServices.ReportProcessing.ReportProcessingException: The Hidden expression for the tablix ‘Tablix5’ contains an error: The expression references the field 'ShowWarehouseOwner', which does not exist in the Fields collection. Expressions can only refer to fields within the current dataset scope or, if inside an aggregate, the specified dataset scope. Letters in the names of fields must use the correct case.
at Microsoft.ReportingServices.RdlExpressions.ReportRuntime.RegisterRuntimeErrorInExpression(VariantResult& result, Exception e, IErrorContext iErrorContext, Boolean isError)
at Microsoft.ReportingServices.RdlExpressions.ReportRuntime.RegisterRuntimeErrorInExpressionAndStop(VariantResult& result, Exception e)
at Microsoft.ReportingServices.RdlExpressions.ReportRuntime.EvaluateStartHiddenExpression(Visibility visibility, IVisibilityHiddenExprHost hiddenExprHostRI, ObjectType objectType, String objectName)
at Microsoft.ReportingServices.ReportIntermediateFormat.TablixMember.EvaluateStartHidden(IReportScopeInstance romInstance, OnDemandProcessingContext context)
at Microsoft.ReportingServices.ReportIntermediateFormat.TablixMember.ComputeStartHidden(RenderingContext renderingContext)
at Microsoft.ReportingServices.ReportIntermediateFormat.Visibility.ComputeHidden(IVisibilityOwner visibilityOwner, RenderingContext renderingContext, ToggleCascadeDirection direction, Boolean& valueIsDeep)
at Microsoft.ReportingServices.ReportIntermediateFormat.TablixMember.ComputeHidden(RenderingContext renderingContext, ToggleCascadeDirection direction)
at Microsoft.ReportingServices.OnDemandReportRendering.InternalTablixMemberVisibilityInstance.get_CurrentlyHidden()

Answer 1

relevant link:https://social.technet.microsoft.com/Forums/en-US/c3e5f557-74f3-4161-aada-ae291d199307/ssrs-2008-r2-equivalent-of-reportexecution2005asmx?forum=sqlreportingservices