"0x0000006 The handle is invalid" error coming in multi-threaded environment

I have created an ASP 3.5 web service which performs thumbnail generation using some Windows APIs. These APIs use Windows HANDLE orHFILE for processing. I am using Asynchronous AJAX in JavaScript to call the web-service.

• Server: ASP 3.5 Web Service
• Client: AJAX in JavaScript

When I am initiating only 1 request i.e. single thread in web service, no error is coming, but
When I am initiating multiple requests i.e. multiple threads in web service, internal APIs listed below are giving errors randomly without any specific patterns. GetLastError returns error code 0x00000006.

Internal APIs: One Set

• _llseek
• _lread

Second Set

• CreateFileMapping
• MapViewOfFile

Notes:

• _llseek and _lread use HANDLE created using

  CreateFile(str, 0x80000000, 0x00000001, IntPtr.Zero, 3, 1, IntPtr.Zero)
  

• CreateFileMapping and MapViewOfFile use HANDLE created using,

  CreateFile(sThumbnailFile, (uint)(0x80000000 | 0x40000000), 0, IntPtr.Zero, 2, 128, IntPtr.Zero);
  

  no error or INVALID_HANDLE_VALUE return code is coming while creating HANDLE.

• This issue is occurring randomly in multiple requests.

• Each HANDLEpoints to a different file.

I have even tested with CRITICAL_SECTION but issue still persists. Please suggest what could be the possible error?