Hi Patrik, I tried your suggestion of adding /usr/lib/libc5-compat to LD_LIBRARY_PATH, but it did not help. Note also that I do not experience any problems with acroread when I start it independently (not from within netscape).
I do use a libc6 version of netscape (version 4.07 from slink), so I could imagine that does not mesh very well with acroread (which uses libc5). But certainly it should work as a helper application (if not as a plugin). Still puzzled, Christian van Enckevort