Exe file which was called from Webservice can not print due to user and credentials

We are working on a Windows 2008 Webserver Edition environment. We need to create bmp files from a report. The whole procedure should be triggered by a webservice.

In  CageTalkerDriversSpool.exe we use this call "!!CB!! 70 Call SalReportPrint( hWndView, sQRP, sVars, sInput, 0, RPT_PrintAll , 0, 0, nFlag )", which calls the  program  reportbuild4.1.exe  in silent mode.  Reportbuild4.1.exe prints the report to the standard printer. As a standard printer we use PDFill image writer as a standard printer, which converts the report to a bmp file. CageTalkerDriversSpool.exe is called from a dynamic link library "CageTalkers.apd". We can see "CageTalkerDriversSpool.exe" for 2 secs in the windows Taskmanager. This is the time it needs to have the report printed.
We get a perfect bmp file as long this is executed by a logged in user.

We want to run this functionality from a webservice!
"CageTalkers.apd" is called from a Webservice, called "CageTalkerExternal" and we can see the program "CageTalkerDriversSpool.exe" in the taskmanager. But the function "!!CB!! 70 Call SalReportPrint( hWndView, sQRP, sVars, sInput, 0, RPT_PrintAll , 0, 0, nFlag )"  can not be executed. It says hanging in the taslkmanager.  We think it needs the proper  windows user to be executed, but we are not sure.

Has anyone an idea how we could solve this problem?

Thanks
Stephan