Adam, thank you so much.
This solved a part of my problem.

Now fileexists works with the UNC path
(\\server\server_c\mypath\myfilename), but it doesn't work with
F:\mypath\myfilename, where F: is mapped to \\server\server_c\.

So I have to convert the path to UNC but that doesn't work with
ExpandUNCFileName.

Example:
F is a mapped networkdrive to \\server\server_e\
If I use
filename := ExpandUNCFileName ('F:\mypath\myfilename')

I get F:\mypath\myfilename in the variable filename allthough it
should be \\server\server_e\mypath\myfilename.
In a testapplication it works correct, but it doesn't in an UDF dll.

For the moment I can make a workarroung by using the UNC path in my
setup, but it would be great also to get it solved without UNC pathnames.

Helmut