ok I found out what the issue is: In a normal window, the logon-popup appears
when accessing the names.nsf (if anonymous access is prohibited). But in a
modal dialog, the logon-popup doesn't come up at all.
If I manually log on to the names.nsf in the same session, everything works
fine in the dialog. I also tried to create a DBLookup to the names.nsf, but
this doesn't seem to have any effect on the AJAX processing...