Just to say that we managed to resolve this issue. We installed host certs on all FST nodes and added the following directives in the /etc/xrd.fst
xrd.tls /etc/grid-security/daemon/hostcert.pem /etc/grid-security/daemon/hostkey.pem
xrd.tlsca certdir /etc/grid-security/certificates/
following this thread Https/webdav on eos5 problem - #12 by esindril
George