Hopla, found it !!
Did a bit of googling on this 'repository prefix' thingy, found this:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=39516 ------- Additional Comment #2 From Michael Valenta 2004-06-16 16:46 -------
This has been fixed in CVSNT. Enable repository prefixes and on the Advanced
page of the CVSNT control panel, enable the "act like a Unix server" option.
Those 2 chechboxes finally did the trick, everything works !
Thanks very much for pointing in the right directory !
Cheers, Jan