Maurizio just helped me. NaviServer requires an addition parameter in the ns/parameters section named "tmpdir":
ns_param tmpdir c:/project-open/servers/$server/tmp
Thanks Maurizio! Frank
see as well: https://openacs.org/bugtracker/openacs/bug?format=table&f_state=8&bug_number=3308&project_id=56106
if there is a windows-specific startup file, it might be a good idea to define the tmpdir there.