While waiting for an answer, I tried several things around my UDFs
and found out, why the error occures.

Don't understand, why it is a problem for the classic server and not
for the superserver, but here is the solution:

This was the function that worked well with superserver (I know, not
very usefull function ;-), but it's just an example):

function donothing (var input : integer) : integer; stdcall;

begin
result := input;
end;

And so it must be, to work with classic server:

function donothing (var input : integer) : integer; cdecl; export;

begin
result := input;
end;

The key was, that I had to replace "stdcall;" with "cdecl; export;"

Don't know why, but it works.

Kind Regards
Helmut