Martijn, it is not a privileges problem.
The account the service runs is an administrator account.

If I pass the local drivename (e.g. c:\ in the directory, it works,
but it doesn't work (e.g. f:\ where f: is a mapped drive or
\\server\server_c\).

There must be a difference between using fileexists in a program and
using it in a dll containing the udf.
I made a small testapplication where filexists works well with unc
pathnames. The same pathname doesn't work in the udf dll.

Very strange.

Helmut