Exceed and offscreen rendering

Hi everyone.
I made a program which renders a 3D scene into a pbuffer/pixmap (if pbuffer aren't supported) in order to export it to a postscript file.
On a RHEL4 (32/64 bits) or whatever distribution may be, it works just fine.
I'm using Exceed when I'm working under WinXP and each time I run my program, more specifically when I export my 3D scene (visualization is OK) using pbuffers or GLX pixmaps, I have this kind of error message:

X Error of failed request: BadImplementation (server does not implement operation)
Major opcode of failed request: 134 (GLX)
Minor opcode of failed request: 5 (X_GLXMakeCurrent)
Serial number of failed request: 22
Current serial number in output stream: 22

Any help will be appreciated.